PROSEA, Introduction to Vegetables
- 1 Definition and species diversity
- 2 Importance of vegetables and vegetable growing
- 3 Botany
- 4 Ecology
- 5 Agronomy
- 5.1 Production systems
- 5.2 Planting materials
- 5.3 Husbandry
- 5.4 Crop protection
- 6 Harvesting and post harvest handling
- 7 Utilization and processing
- 8 Genetic resources and breeding
- 9 Prospects
- 10 Authors
Definition and species diversity
Choice of species
Vegetables form a large and diverse commodity group. They are considered a distinct group, not because they have botanical features in common, but largely because of the way in which they are grown and their produce is used. Vegetables are usually cultivated intensively in "gardens" and consequently are part of horticulture. They are usually consumed in combination with starchy staple foods, sometimes used in small quantities to contrast with other foods in taste or to add flavour to a meal.
A vegetable as a product or commodity may be defined as a usually succulent plant or portion of a plant which is consumed as a side dish with the starchy staple. All vegetable crops share certain common characteristics but very few completely fit any definition.
In this volume approximately 100 important vegetable species (sometimes subdivided into cultivar groups as in Allium cepa L., Brassica oleracea L. and Brassica rapa L.) are described in 86 papers in Chapter 2, and summary data are given on 125 minor vegetables in Chapter 3.
The selection of species is somewhat arbitrary because it is impossible to define vegetables in a way that clearly sets the boundaries with other commodity groups. Extensionists, farmers and agronomists generally group all crops in the category "vegetables" which show similarities in cultivation methods with the familiar vegetables. For example, melon and watermelon are commonly classified as vegetables because of the resemblance to cucumber; traders and consumers, however, generally classify them as fruits. In other cases such as garlic, chives or capsicum pepper, the borderline with "spices" is vague, and inclusion in the vegetables is a matter of convention or convenience.
The choice of species for this volume, however, is above all a function of the commodity grouping adopted for the Prosea handbook (Jansen et al., 1991). Important leguminous vegetables (French bean, kidney bean, lablab, mungbean, pea, soyabean) are not described in this volume; because the mature, dry seeds of these species are also used, they have been assigned to Prosea volume 1: "Pulses" (1989). Immature fruits of jackfruit and papaya, and the leaves of Gnetum gnemon L. are important vegetables, but these species are described in Prosea volume 2: "Edible fruits and nuts" (1991). Sesbania grandiflora (L.) Poiret can be found in Prosea volume 4: "Forages" (1992). Rattans and bamboos are dealt with in volumes 6 and 7. For many root and tuber crops with vegetable uses (cassava, potato, sweet potato, taro, yam bean) reference is made to Prosea volume 9: "Plants mainly producing carbohydrates". Zea mays L., and thus vegetable maize (including baby corn, sweet corn and young corn cobs) is described in Prosea volume 10: "Cereals", and Centella asiatica (L.) Urb. in Prosea volume 12: "Medicinal plants". Coriander and mint are included in Prosea volume 13: "Spices". Prosea volume 15: "Lower plants" deals with edible fungi, ferns and other lower organisms. Many plant species other than those mentioned above yield vegetables as a by product, though they themselves used primarily for other purposes. Approximately 800 such species are listed in Chapter 4 of this volume, with a tentative reference to the volume where more detailed information, including the vegetable aspects, can be found.
Domestication and introduction
It is estimated that in the course of time and on a worldwide scale, 1500-2000 plant species have been used as supplementary food. For South-East Asia, the number is close to 1000 species (Terra, 1966; Grubben, 1977; Siemonsma & Aarts van den Bergh, 1989; Jansen et al., 1991).
Originally they were gathered from the indigenous vegetation, but soon some form of "in situ" protection led to primitive cultivars in at least 500 species. The most suitable ones (about 200 species) were moved closer to dwellings and cultivated in home gardens or mixed with field crops to obtain a more reliable supply for home consumption. About 80 of these 200 cultivated species proved to be sufficiently profitable for labour intensive market garden production. Only 20 species proved suitable for the highly intensified, protected cultivation systems as practised in western countries.
Among the 225 "primary use" vegetables described in this volume, the species cultivated for the market or for home consumption (about 120) figure prominently. Nevertheless, more than 100 wild species are described as well, including a large number of weedy companions of the field crops.
The above description of the domestication process suggests a harmonious situation with a well balanced, well adapted assortment of vegetables consisting of the best elements of the indigenous vegetation, as a result of a long process of selection and elimination. It may, therefore, come as a surprise that more than 80 of the 130 cultivated species have been introduced - deliberately or accidentally - into the South East Asian region. Most of them only occur in cultivation, although some introductions from South and Central America have proven to be well adapted and have become naturalized (e.g. Cosmos caudatus Kunth, Limnocharis flava (L.) Buchenau). About 50 species from the Asian mainland and other tropical areas have crept into the region, brought along by missionaries, merchants and settlers. These vegetables have been assimilated in a gradual process during many centuries.
The colonial past led to the introduction of about 30 species of temperate origin, more or less suitable for cultivation in tropical highland areas (e.g. white cabbage). Some have succeeded in obtaining a strong position in the commercial sector in South-East Asian countries. New introductions still emerge from time to time. Popular Japanese vegetables such as "gobo" (Arctium lappa L.), "mitsuba" (Cryptotaenia canadensis (L.) DC.) and "fuki" (Petasites japonicus (Sieb. & Zucc.) Maxim.) now occupy small niches in South East Asian highland areas, satisfying the demand of a foreign clientele but with the potential to be assimilated by the well to do part of the local population as exotic vegetables with a high social value. The question arises whether one is witnessing here a harmonious addition to the assortment of vegetables in South-East Asia or the symptoms of an unequal battle between the highlands and the lowlands, between temperate and tropical vegetables.
From ancient times vegetables have been produced in the vicinity of human dwellings because in contrast to cereals and pulses it is difficult to transport and store these bulky and perishable products. In modern times, as a result of improved roads to consumer markets in urban centres, vegetable production areas have developed where land facilities and climatic conditions are good.
The cultivation of vegetables from temperate areas, often called "exotic", "European" or "highland" vegetables, has become very widespread in the recent past. For instance, about half of the registered vegetable production in Java (Indonesia) consists of highland vegetables. These highland vegetables have partly replaced the traditional "tropical" vegetables in local diets, especially in urban areas. Important production areas for highland vegetables can be found in Indonesia at Puncak, Sukabumi and Lembang for the urban centres of Jakarta and Bandung (West Java); at Dieng for Yogyakarta and Semarang (Central Java); at Tretes and Batu for the markets of Surabaya and Malang (East Java); further at Brastagi for the market of Medan (North Sumatra) and for export to Singapore. Similar highland vegetable production can be found in Malaysia in the Cameron Highlands, in Thailand at Chiang Mai, in the Philippines at Baguio, and in Vietnam at Dalat.
In the lowlands, large concentrated areas of vegetable production have also developed. Often these production areas are situated close to big cities, e.g. Jakarta, Surabaya, Bangkok, HoChiMinh City, and include the more traditional "tropical" vegetable types, especially the easily perishable leafy vegetables. In some cases they are situated hundreds of kilometres from urban markets. The shallots and capsicum peppers grown in Tegal/Brebes (Central Java) are traded all over Java. However, in general the production of vegetables in the lowlands is more thinly spread over larger areas, and this impedes extension to and organization of farmers, and forms a handicap for traders.
The high productivity of temperate vegetables such as white cabbage, carrot and tomato, led to the prejudice that they are superior to indigenous species, whereas this is simply the result of prolonged and expert selection by the western based horticultural breeding industry during a period that the research and development efforts in South East Asian countries were still allocated to other priorities, mainly to self sufficiency in staple foods. With the explosive development of large urban centres, these productive temperate vegetables are a convenient answer to the increased dependence on market products. However, there are disadvantages as well: they are often cultivated in environmentally sensitive areas (slopes, watersheds); their cultivation depends more heavily on fertilizers, pesticides and a seed supply from abroad; the highland vegetables are on average more expensive, and the most popular highland vegetable, i.e. white cabbage, lacks the vitamin A which makes traditional green leafy vegetables so nutritious. It is generally acknowledged that of all crop plants in the tropics, indigenous vegetables have been long neglected in agricultural research, and much remains to be done to collect and study their diversity and to improve yield and quality. This is especially true for the lowland humid tropics. Even now there is more "adaptation" breeding work going on to try to invade the lowland areas with heat tolerant "highland" vegetables including Irish potato than to develop the lowland vegetables in their natural surroundings.
Advantage should be taken of the potential to grow temperate vegetables (enrichment of assortment, off season export), but substitution and the disappearance of traditional vegetables will be regretted in the long run. The South East Asian countries are still at the lower end of the recommended vegetable consumption. Demand for vegetables shows high income elasticity, and with increased economic performance, demand, not least for more diversity, is expected to grow sharply. Most South East Asians live in lowland areas, most indigenous vegetables are lowland species.
Importance of vegetables and vegetable growing
As the product of an intensive form of agriculture, vegetables are becoming increasingly important as cash crops for urban and export markets, with a great potential to improve nutrition and health of the rural and urban poor, as well as to increase their incomes and provide better employment opportunities.
Statistics on vegetable production are scarce and not very reliable. Most countries only record the acreage and production of the major commercial vegetables and ignore the many minor commercial crops and the very important part produced for home consumption. FAO statistics which are based upon country statistics are misleading in this regard. Food consumption surveys offer the clearest picture of the types and quantities consumed. A comparison between countries is also misleading because the crops or food commodities considered as "vegetables" differ from country to country. Statistics are often based on monthly observations on planted and/or harvested area, and consequently the acreage of vegetables with repeated harvests over several months is much overestimated and the yields are underestimated when these monthly records are computed into an annual list. To give a striking example: 20 ha of kangkong which is ratooned once per month during 8 months, yielding 120 t per ratoon, may appear in the statistics as: kangkong 160 ha; 960 t (6 t/ha), which should be corrected to: kangkong 20 ha; 960 t (48 t/ha). Another problem in collecting statistics is the estimate of areas per crop in the case of mixed cropping.
Based on recent inventories for this book of a large number of urban markets in South East Asian countries, an attempt has been to indicate the occurrence and relative importance of all major "primary use" vegetables described in this volume (Table 1). The most important "secondary use" vegetables (out of the 800 listed in Chapter 4) are added at the end of the list.
In 1989, the world production of vegetables reported by FAO (1990) was 433 940 000 t, the South East Asian production (Indonesia, Malaysia, Papua New Guinea, the Philippines, Thailand, Vietnam) was 10 674 000 t. On a global scale, production per capita per year rose during the 1980s from 78.6 to 83.4 kg, in South East Asia it fell from 32.1 to 28.1 kg. It is a matter of considerable concern that in many tropical countries, South East Asian countries included, vegetable production seems to be lagging behind the rate of population growth. For some countries such as the Philippines and Thailand, figures even indicate a decrease in absolute terms over the last decade. Urbanization and changes in landuse at the periphery of the big cities, and a backward market system might be the main causes of this phenomenon. In food deficit areas in Vietnam, the difficulty of supplying minimum energy requirements through supplementary imports from food surplus areas has contributed to a strong emphasis on rice self sufficiency. This has been accompanied by a decline in the production of other subsidiary food crops such as vegetables, which are particularly important in improving the dietary balance and variety (FAO, 1988). The FAO production estimate (kg/capita per year) for Indonesia is rather low (18.8 kg in 1989) compared with other countries in the region. This suggests the existence of an important informal production circuit (small family gardens) not covered by the FAO data. Household expenditure surveys (BPS, 1983) suggested that in 1980 the total production (and consumption) of vegetables in Indonesia was at least twice the FAO statistics.
Vegetables as a group account for 5-6% of the total value of agricultural production in South East Asia, exceeding the value of major secondary food crops such as maize, cassava, groundnut, soyabean, sweet potato or mungbean.
Although the productivity of most vegetable crops is still rather low, incomes per unit area are usually relatively high. Net revenues per hectare from shallot, pepper and tomato in Indonesia have been reported to be 3-5 times higher than those from rice. The import and export of fresh vegetables in South East Asia is still very limited as a percentage of total production. Trade of fresh produce between countries within the region is mainly linked to the supply of large urban centres like Singapore (e.g. cabbage from Brastagi, North Sumatra, Indonesia, and from Cameron Highlands, Malaysia), whereas there is some off season production of temperate vegetables for the Japanese market (e.g. Arctium lappa L. and bulb onions from northern Thailand).
Vegetables are consumed because they are tasty and healthy. They add variety and flavour to the diet. There is little chance of malnutrition occurring in families consuming enough vegetables. Vegetable consumption may, therefore, be considered as an important economic factor in a society because it improves health and working capacity. Vegetables are generally low in energy and dry matter content, but most important as sources of protective nutrients, especially vitamins and minerals. Vegetables (together with fruits) are the most important source of vitamin A, which is deficient throughout South East Asia where rice based diets predominate, blinding thousands of children annually (Oomen & Grubben, 1978). Although the protein content of vegetables is considered unimportant in developed countries, it appears to be highly significant in countries with an overall deficiency in proteins. Vegetables also provide fibre in the form of cellulose which aids the digestion of other foods and stimulates and cleans the intestinal canal.
The nutritive value of vegetables varies but is usually greatest in those eaten raw. In Indonesia and Malaysia, but also in other parts of the region, it is customary to eat a large variety of vegetables raw. This so called "lalab" or "ulam" can also be eaten after being blanched (immersed in boiling water for several minutes), making the vegetables softer but without losing their consistency. They are often consumed in combination with a groundnut sauce or with coconut milk. Also in Chinese cuisine, vegetables are not boiled thoroughly, but usually stewed or fried in oil (strongly enhancing the availability of carotene). In New Guinea, vegetables are often wrapped in banana leaves and baked in hot ashes or between hot stones. The disadvantages of certain preparation techniques leading to overcooking can easily be overcome by slightly increased consumption, and therefore emphasis should be on averting underconsumption instead of on preparation methods.
Consumption in South East Asia can be roughly put at 80% of the production in kg/capita per year, and (after correcting the production data for Indonesia) averages about 90 g/capita per day or 33 kg/capita per year. This compares favourably with consumption levels in Africa and South America, but is only 45% of the consumption in the developed countries. It is also much lower than the daily intake of 150 g/capita recommended by nutritionists as a target, provided that one third of this amount derives from leafy vegetables.
A FAO/World Bank study of the agricultural and food situation in Vietnam (FAO, 1988) revealed that the nutritional status of the Vietnamese people is extremely low. Vietnam ranks among the most deprived countries in Asia. Overall there is a high prevalence of protein energy malnutrition (PEM), endemic goitre, iron deficiency anaemia, vitamin A deficiency and other micronutrient deficiencies. The nutritional status is most severe in the northern and central coastal regions and in hilly and mountainous areas, not necessarily in areas of the highest population density.
Nutritional needs are subject to regional differences. According to FAO/WHO standards for East Asia (FAO, 1972), the average daily requirements of an adult man (55 kg) are: energy 10 600 kJ, protein 46 g, carotene (pro vitamin A) 1.5 mg, thiamine (vitamin B1) 1.0 mg, riboflavine (vitamin B2) 1.5 mg, niacin 17 mg, vitamin C 30 mg, Ca 500 mg, Fe 9 mg. This information can be related to the data on the nutritional composition of individual vegetables as given in the entries in this book, but it must be stressed that the composition of foodstuffs varies widely as a result of environmental factors, varietal differences, cultural practices, harvesting stage of the plant, methods of storage, processing and preparation. Data are expressed on a fresh weight basis, and are therefore strongly influenced by the dry matter content.
Characteristics of the vegetable sector per country
The situation of vegetable production and consumption in six South East Asian countries is outlined briefly below.
The Lembang Horticultural Research Institute (LEHRI) prepared a profile of the vegetable production and consumption situation of Indonesia, based upon the available statistics (van Lieshout, 1990). Production statistics are defined by the commercial production of 18 major vegetable types. The commercial production of 1988 is presented in Table 2. Subsequently it was calculated that the total commercial area needed for the year 2000 would be 1 148 000 ha, based on the following assumptions:
- 7% annual growth of commercial demand as a consequence of population growth (2%), increase of incomes (3.5%) and urbanization (1.5%);
- 7% annual growth of commercial supply as a consequence of better technology (2%) and area enlargement (5%).
The seven most important vegetables in terms of total production in 1988 (each more than 200 000 t) were respectively cabbage, hot capsicum pepper, potato, shallot, cucumber, yard long bean, and caisin/Chinese cabbage. The total farm gate production value was about 1250 billion Rupiah or US$ 735 million. The commercial production of 4 170 400 t mentioned in Table 2 is only 47% of the consumed quantity, which leads to the conclusion that the many non registered commercial vegetables, each of them of minor importance, together with the numerous species for home consumption, account for 53% of total consumption.
The annual export (1986: 22 900 t, mainly potato and cabbage, valued at US$ 10.4 million) and import (1986: 3400 t, mainly garlic, shallot planting material and dried hot pepper, valued at US$ 1.5 million) amount to less than 1% of the total production. The gross intake per capita per year is 46.7 kg, which means 37.4 kg net if 20% is deducted for waste between farm gate and consumption. A daily intake of 102 g per head is reasonably high if compared with other tropical countries. But the consumption is not evenly spread. High income classes consume more vegetables than low income classes, and consumption in West Indonesia, especially West Java, is much higher than in the eastern part.
At present the production for home consumption is still very important, but it is expected that in the coming decades it will partly be replaced by commercial production. The gross urban consumption in 1990 (43.7 kg/capita) was lower than recorded in 1987 (47.7 kg/capita) whereas the gross rural consumption increased slightly (from 47.4 to 48.0 kg/capita). The most likely explanation is reduced availability in urban areas, possibly caused by an obsolete marketing system.
The data of 1990 reveal that boiled leafy vegetables (kangkong, amaranth, caisin, cabbage, cassava leaves, etc.) constitute 40% of the total quantity consumed; boiled non leafy vegetables (yard long bean, eggplant, chayote and other cucurbits, carrot, young kidney beans, potato, bean sprouts, young jackfruit) constitute 41%; spice vegetables (shallot, garlic, hot capsicum pepper) constitute 13% and raw (salad or "lalab") vegetables such as cucumber and many leafy vegetables constitute 6%.
From the scarce statistics available it may be deduced that the range of vegetable crops lies between Thailand and Indonesia. Highland vegetables are produced in the Cameron Highlands, and lowland vegetable production is scattered everywhere in the coastal plains. The seven most important crops in acreage in Peninsular Malaysia registered in 1988 are watermelon 2400 ha, hot capsicum pepper 1400 ha, cucumber 1300 ha, cabbage 800 ha, leaf mustard 800 ha, kangkong 700 ha and amaranth 500 ha. The annual export of vegetables (mostly highland) to Singapore is considerable: about 120 000 t valued at 61 million Malaysian Ringgit or about US$ 24 million (1989). Onions, shallots and garlic are imported. No statistics are available on vegetable production for home consumption.
Papua New Guinea
The vegetable types grown in Papua New Guinea are essentially the same as those in Irian Jaya and other provinces of East Indonesia. Popular highland vegetables are garlic, potato, tomato, cabbage, carrot and welsh onion. Cucumber is very popular and is grown both in the highlands and in the lowlands. Winged bean is important for its tubers, seeds and young pods. Popular leafy vegetables are aibika, chayote tops, sweet potato tops, amaranth and rungia. There is no significant import or export of vegetables. Numerous indigenous species are collected or grown in home gardens but no statistics are available.
Inventories of vegetable markets in Luzon, the Philippines, give the impression that fruit vegetables (cucurbit and solanaceous fruits) are in ample supply, but that leafy vegetables play a smaller role in the diet than in other South East Asian countries. The registered area of vegetables in 1987 was 142 348 ha (including potato and ginger, excluding dry beans and peas). The recorded production was 1 008 960 t, the average yield 7.1 t/ha. The most important vegetables in cultivated area are tomato (18 000 ha), eggplant (16 000 ha), onion and shallot (7000 ha). The most important vegetables in tonnage of yield are watermelon, tomato, eggplant, cabbage, and pumpkin. The most important fresh vegetable for export is onion (7400 t with a value of US$ 2.6 million) but many other fresh and preserved vegetables are exported.
Two main vegetable production areas are distinguished. In the hot humid lowlands, hot season vegetables are produced the year around. During the rainy season the types most vulnerable to heavy rains like shallot, tomato and Chinese cabbage are grown to a lesser extent. In the northern part, the cool dry winter season is suitable for the more temperate types such as cabbage, tomato, and garlic. Area, yield and production of 22 major commercial vegetable species are presented in Table 3.
The import of vegetables is insignificant, the export is becoming increasingly important. In 1989 the export of garden crop products amounted to 233 225 t with a value of 3.6 billion Bahts or US$ 144 million. The most important of the many vegetable commodities exported were baby corn and bamboo shoots. No statistics are available on vegetable production for home consumption or on consumption per commodity.
The Ministry of Agriculture recorded an area of 242 800 ha for vegetables in 1988 (private 208 900 ha; cooperative 33 300 ha; state 600 ha) with an average yield of 12.0 t/ha. The total production was 2 909 000 t. The projected annual growth rate for the period 1986-1990 was 6% for acreage, 2.7% for yield and 8.9% for production. However, production statistics are unreliable. Perhaps half of the vegetables for home consumption and sale in local markets are grown on private plots, not monitored by the state (FAO, 1988). There are two distinct production seasons, the "winter" season (October - February) and the "summer" season (April - August). Yields of vegetables reach 20 t/ha in Lam Dong and some Mekong Delta provinces, 18 t/ha in HoChiMinh City (15 000 ha) and 13-14 t/ha in Hanoi (13 000 ha). In the past, the government has taken little interest in small scale private sector production, which receives almost no attention from research, extension or other officially sponsored support services. Nevertheless, horticulture accounts for a large part of the value of agricultural production, consumption and cash income for farm families. Fresh or processed vegetables are a potentially important export crop. The export of fresh vegetables (cabbage, carrot, kohlrabi, cucumber, onion, cauliflower) and of preserved products (pickled cucumber) was about 12 000 t in 1988 (with a value of US$ 1.9 million). The export of watermelon (1988: 11 300 ton with a value of US$ 1.7 million) is as important as all other vegetables together.
Priority is given to expansion of production during the winter/spring season, especially in the Red River Delta. Input requirements and transport and marketing difficulties are the major constraints. Vegetables reportedly absorb about one half of pesticide imports. Insecticide resistance has become a problem in the production of leafy vegetables. The major issues for vegetable production of the big cities Hanoi and HoChiMinh City are local self sufficiency and reducing the seasonality of production. Before 1975 HoChiMinh City had only 10% of its present vegetable area and depended on shipments mainly from Dalat, 300 km away in the highlands of Lam Dong Province. This trade declined as transport deteriorated and private marketing activities were suppressed, and HoChiMinh City was forced to strive for local self sufficiency in lowland vegetables. The cool climate of Dalat is suited for temperate type vegetables (cabbage, kohlrabi, carrot, potato, tomato, onion, garlic, etc.) especially during the off season (April, May, September and October). Similar opportunities exist for cool season vegetables during the summer period in the mountainous areas north west of Hanoi.
The heterogeneity of the commodity group vegetables is well illustrated by the fact that the 225 species described belong to approximately 60 different plant families: vegetables occur throughout the plant kingdom. Nevertheless, some families figure prominently with respect to number of vegetable species as well as economic importance: Compositae and Cruciferae, mainly as leafy vegetables, Cucurbitaceae and Solanaceae, predominantly as fruit vegetables.
The proper naming of plants is extremely important, because it enables repeatability and use of scientific methods. Taxonomy provides such a naming service and is therefore an important biological science. Many taxonomical problems are still unsolved. Few major genera of economically important vegetable families have been revised in their entity within the last 50 years. But in genera of minor importance such as Spilanthes Jacquin or Sonchus L., the lack of linkage of data to well defined taxa also makes much of the information useless.
The vegetables comprise some genera, Brassica L. in particular, showing the most bewildering array imaginable of types created by man and nature over centuries during the selection of cultivated plants. Brassica vegetables span a range of morphotypes comprising succulent modifications of leaves, stems, roots, buds and floral parts. Taxonomically they have been variously classified, leading to much confusion because the classic taxonomy is primarily intended for wild taxa. Closely related types were often classified as subspecies or varieties (formal classification under the International Code of Botanical Nomenclature), but the most recent approach is to distinguish cultivars and to group these in cultivar groups (the informal "open" classification guided by the International Code of Cultivated Plants). Where workable cultivar group classifications have been developed (Allium cepa L., Brassica oleracea L., B. rapa L.), these are being followed or even promoted in this volume. Because of its illustrative value, the nine contributions on particular Brassica crops are preceded by a genus article (Brassica L.) outlining the taxonomic and cytogenetic basis of the present day classification.
The taxonomy of cultivated plants is developing: as yet there is no worldwide accepted system for naming and classification, but proposals are being discussed.
Vegetables are often classified according to morphological criteria such as plant parts used or growth habit. Plant parts used as vegetables comprise anything from whole leaves (kangkong, amaranth, welsh onion) to petioles (rhubarb), fruits (cucurbits, solanaceous fruits, leguminous pods), flowers (cauliflower, broccoli), stems (asparagus, bamboo shoots) and stem tubers (above ground such as kohlrabi, or underground as in yam bean, potato), seeds (cucurbits, young leguminous seeds), storage roots (carrot, edible burdock) or bulbs, which are swollen leaf sheaths (onion, garlic, shallot). In one case, the stem tuber is the result of a symbiosis of plant host (Manchurian wild rice) and a fungal parasite. If the 225 or so vegetable species described in this volume are classified according to plant parts used, leafy vegetables are by far the most frequent (about 60%), whereas fruit, flower, stem, root and seed vegetables account for about 15% of the species. The remaining 25% consist of multipurpose vegetables, characterized by more than one edible part. The record is probably held by the winged bean (Psophocarpus tetragonolobus (L.) DC.) with edible young pods, young seeds, flowers, leaves and tubers. Multipurpose vegetables may be useful for home gardens, but it should be kept in mind that the use of one part often precludes, or at least negatively affects the yield of the other; multiple use of the same plant is therefore not very common in commercial production.
In areas with cold winters, annual crops dominate the agricultural scene, but as one moves from higher latitudes towards the equator, woody perennials become more important as sources of food, and this also applies to vegetables (Cannell, 1989). About 25% of the species treated in this volume have a woody growth habit, shrubs being more frequent than trees. They play a relatively important role in home garden production as they are a more permanent and flexible source of supplementary food, and can serve other purposes simultaneously.
Growth and development
Knowledge of crop growth and development and insight in the eco physiology should help the grower to manipulate the crop and its environment so as to achieve the optimum yield of the desired plant part. Most vegetable crops do not pass through their complete life cycle under field conditions because they are grown for their vegetative parts, or young, immature generative parts. For leaf vegetables, flowering and fruiting are to be avoided or delayed. For bulb and tuber vegetables, the formation of the storage organs is a delicate phase in the succession of growth and development processes. When flowers or fruits are the useful product, the objective is to channel as much energy to these generative plant parts. Although flowering and fruiting are often undesirable in vegetable production, the potential to complete the life cycle is important for seed production.
Information on the growth and development of vegetables is very limited except for a number of species of temperate origin. The sequence of germination, vegetative growth, development of storage organs or generative development (flower initiation, flowering, pollination, fruiting) is highlighted in the species treatments.
The area covered by Prosea lies between 20°N and 10°S. It consists mainly of tropical lowlands, but also has large areas at medium to high altitudes.
In agriculture, the choice of crops and cropping systems is mainly determined by interactions of ecological factors (climate, soil) and management variables. Horticulture is an intensive form of agriculture, usually on small acreages, in which restrictions imposed by adverse climatic factors and poor soils can often be overcome by intensive management practices. Commercial vegetables are not always grown on the site and at the time which are ecologically the most appropriate, because the ultimate motive for the farmer is profit, not yield. A short distance to the market or a high price during the off season often compensate for a lower yield or higher production costs. In Indonesia the altitude of the production area and the distance to the city markets (transport facilities) appear to be by far the most important factors determining which species farmers choose to grow.
The climates of South East Asia are of the monsoon type. Monsoons are seasonal winds blowing moist air from the sea to the heated land mass bringing heavy rains during the hot season, and blowing air from the land to the sea during the cold season. In Indonesia and Malaysia, situated close to the equator, the dry south east trade wind from Australia causes a dry spell from May to October. This wind turns to the north above the equatorial zone and takes up much moisture above the ocean. It is known as the south west monsoon in Thailand and neighbouring countries and causes the rainy season from May to October ("summer"). The inverse happens from December to February when the north east monsoon causes the dry season ("winter") in Thailand but brings rain as the west monsoon in Indonesia.
Temperature is the most important climatic factor for vegetable production. In the lowlands near the equator the average daily temperatures are generally about 27°C the year around, the differences between a hot and a cold season becoming more pronounced northwards. In northern Vietnam the average temperature from November to April is only 16°C. In these areas the summers, from May to October, are very hot and subject to typhoons.
In mountainous areas the temperature drops by about 1°C per 160 metre increase of elevation and the difference between day and night temperature broadens. The occurrence of micro climates is quite common. Large variation in rainfall, temperature, radiation and wind may be observed between areas at relatively short distances from each other. A large part of the vegetable production is in the highlands. Since temperature is the most important factor determining the choice of vegetables that can be grown at a certain altitude and since the transition between highlands and lowlands is gradual, there is a need for a practical classification into ecological zones. An example is the empirical classification made by the Lembang Horticultural Research Institute in Java, Indonesia (Buurma & Basuki, 1989). This classification, derived from a statistical databank of 18 commercial vegetable crops, is fashioned in such a way that over 70% of the area of the typical lowland vegetables (cucumber, kangkong, yard long bean) comes into the lowland zone and over 70% of the typical highland vegetables (cabbage, carrot, potato) in the highland zone, with only a minimum of overlap in the medium elevation zone (Fig. 1). Depending on the precision desired, the classification may comprise three zones (lowland < 200 m; medium land 200-700 m; highland >700 m), or an even finer classification into four zones by subdivision of the medium land zone into a low medium (200-450 m) and a medium high zone (450-700 m). This method of defining the ecological zones is useful for the interpretation of statistical data and the results of multilocational cultivar trials.
The lowland and low medium zones of Java (below 450 m) are characterized by high maximum day temperatures (30-27°C) and night temperatures (25-22°C) and high light intensity. The main soil types are alluvial clay in the coastal plains and latosol at higher altitudes. The medium high and highland zones (above 450 m) are characterized by maximum temperatures below 27°C, a larger variation between day and night temperature, a lower light intensity because of cloudy weather and a high air humidity. The main soil types are andosols and grumosols.
Most vegetables are dominantly present in one of the ecological zones, but they overlap. Welsh onion, red kidney beans or tomato apparently have a large optimal temperature range, since they are grown from sea level up to high altitudes. Hot capsicum pepper is a typical lowland vegetable (79% < 450 m) but yet it occurs also in the highlands, even up to 1800 m. Typical highland cultivars of capsicum pepper do not perform well in the lowlands and vice versa. The distinction between highland and lowland cultivars, or, in the higher latitudes of northern Thailand, Philippines and Vietnam, between "summer" (hot season) and "winter" (cool season) cultivars is well known for some important commercial vegetables such as cauliflower, white cabbage, Chinese cabbage, tomato, and capsicum pepper.
The variation in daylength is a less important climatic factor in the area close to the equator, but it is of increasing importance further north. At 10°N (southern part of the Philippines, Thailand and Vietnam) the daylength varies from about 11.30 h to 12.40 h and at 20°N (northern part of the Philippines, Thailand and Vietnam) from about 10.50 h to 13.20 h. The distinction between a "summer" and a "winter" becomes tangible above 10°N, by variations in the photoperiod and in the total daily radiation. Some crops are very sensitive to daylength variations, a nice example being okra (Siemonsma, 1982). These daylength effects will be dealt with in the species treatments.
In South East Asia vegetables can be grown year round, provided that enough water is available. As a rule of thumb, actively growing leafy vegetables need 6 mm (6 liter per m2) daily and other vegetables 4 mm. In the absence of rain or irrigation, the moment growth will be retarded and drought damage will occur depends on the type of crop, the soil properties and the cultural practices. In general, the yields obtained from commercial vegetable production are higher in the dry season with irrigation than in the rainy season without irrigation. The reasons for this yield depression during the rainy season are the deficient radiation by cloudy weather and the damage resulting from diseases.
Farmers are bound to a certain land area and normally have little opportunity to choose a soil type suitable for a certain crop. In general, a good soil for vegetable crops should have the following properties:
- Good structure: this means that the soil must be durably friable and stable, providing adequate water retention and aeration. A good soil is a proper medium for high microbiological activity and for undisturbed root development to a depth of at least 60 cm.
- High chemical fertility: the soil should contain reserves of essential nutrients, a sufficient amount of which must be readily available in the soil moisture. The rate of growth and production depend on the element in shortest supply ("critical element").
Ideal soil properties are identical for almost any vegetable crop. However, vegetable species and even cultivars of the same species differ in their yielding ability under adverse conditions, e.g. shallow, wet or dry, acid or saline soils. A high soil salinity (electrical conductivity > 3.0 mmho/cm) is a serious restriction for satisfactory yields of most vegetables; however, tomato, broccoli, cucumber and pumpkin are reasonably salt tolerant. Vegetables with a shallow root system like onion, cabbage and kangkong are much more susceptible to drought than deep rooting species like tomato, watermelon and asparagus.
The predominant soil types in South East Asia are andosol and latosol (both of the sandy loam type) and alluvial clays. Light soils have the advantage of easy tillage, adequate drainage and aeration, provided that the organic matter content is sufficiently high. Clay soils have the advantage of a better water holding capacity and higher natural fertility. With good cultural practices, most vegetables will give satisfactory results on a wide range of soil types. Yet some crops such as cabbage and garlic prefer a heavy soil, whereas others like asparagus, carrot and radish prefer a light soil. Table 4 gives characteristics of two typical soil types used for vegetable production in the lowlands of Indonesia (Titulaer, 1991).
In intensive vegetable cultivation, the lack of chemical fertility is not perceived as the most serious limiting factor because amendments with manure and/or inorganic fertilizer are relatively easy. Lacking adequate recommendations, most farmers in South East Asia rely on their own experience in the application of manure and mineral fertilizer. Unfortunately, in many cases their practice is injudicious and unbalanced, supplying too much of one element and not enough of another. Many farmers use heavy N dressings in the form of cheap urea, which causes fast vegetative growth, making the plants susceptible to diseases and damage. Too much fertilizer means high costs of inputs and pollutes the environment. Sustainable soil conditions should be aimed at by establishing a sufficiently high level of basic fertility, and giving an appropriate fertilizer gift to compensate for the expected uptake per crop.
The pH of the soil influences the availability of nutrients and also the soil structure. If the soil is very acid (pH water < 5.5), the choice of the crop will be limited to only a very few species such as shallot or watermelon, and will certainly not be suitable for cabbage. Crops on acid soils often suffer from Mg, Ca or P deficiencies, or from Mn and Al toxicity. Liming with slakes (or preferably with dolomite) is useful at a rate of about 2 t/ha per crop until a level of pH 6-6.5 has been reached. On acid soils, it is recommended not to use too much of acidifying fertilizers such as ammonium sulphate or urea.
Physical soil properties and organic manure
The volume of an ideal soil profile consists for one third of each of the three elements: solid mass, moisture and air. This constitution will guarantee adequate aeration, water holding capacity, drainage and biological activity. Organic matter has the characteristic that it reduces the compactness of heavy soils and increases the water holding capacity of light soils. Light sandy soils should contain at least 4% organic matter, which corresponds to 2% C. For heavy clay, about 2% organic matter content (1% C) is needed. At these levels the yearly losses of organic matter are approximately 5 t/ha. This loss can be compensated by an application of about 10 t/ha of manure, but higher doses (up to 80 t/ha) are often practised for intensive vegetable production.
The low productivity of many vegetable crops in the tropics is largely due to the lack of research attention, aggravated by agro economic constraints such as insufficient farm capital, inadequate transportation, and dramatic price fluctuations. Increased agronomic research attention for vegetable production translates itself into:
- knowledge of appropriate production systems;
- improved cultivars and the availability of high quality seeds;
- appropriate cultural and management techniques;
- adequate control of diseases and pests.
The production systems for vegetables in South East Asia fall into four main groups, based on land use and the level of inputs. In this section the relative importance of each of these for home consumption and for marketing is estimated and expressed as a percentage. However, in each country and region the situation will be different.
Collection of weeds and wild plants
The picking and gathering of vegetables from the wild vegetation (mostly leafy vegetables but also berries, roots, etc.) is still important in rural areas. While weeding or gathering firewood, the women often pick these edible plants (pot herbs) for the preparation of their meals at home. In the fields they also practise selective weeding, the useful weed plants being spared.
About 100 species of the 225 primary use vegetables described in this volume are weeds or wild plants. Possibly they account for about 15% of home consumed and 5% of the marketed volume. With the increase in the population, the urbanization and the specialization in professional activities, this type of food collection will decrease further.
A considerable part of the vegetable production, estimated at about 30% of the home consumed and 10% of the market vegetables, is derived from the compounds close to the houses. Annual vegetable types often encountered in the compounds are amaranth, caisin, kangkong, yard long bean, lablab, winged bean, cucumber, bottle gourd, pumpkin, bitter gourd, chayote, capsicum pepper, eggplant, cassava (leaves). Climbing types of leguminous species or cucurbits are important because of their ability to occupy open places. However, trees and shrubs dominate the home gardens almost everywhere because they are a more permanent and more flexible source of supplementary food than annuals, they have been less successful in migrating to more commercial market gardens than herbaceous crops, and they serve many other purposes better than annual herbs such as providing shade in the compound (Parkia speciosa Hassk., Archidendron jiringa (Jack) Nielsen, Ficus spp.), serving as a hedge (Sauropus androgynus (L.) Merrill, Polyscias spp.), providing living support for other plants (Moringa oleifera Lamk), as ornamentals (Polyscias spp.), medicinal plants (Gynura procumbens (Loureiro) Merrill), or as a source of forage (Sesbania grandiflora (L.) Poiret) or fuelwood (Moringa oleifera Lamk).
Home gardens are characterized by a great diversity of useful plant species growing in a herbaceous layer near the ground, and various layers of canopies of shrubs and trees. Traditionally, home gardens in rural areas are very rich in fruits, spices and medicinal plants. Vegetables constitute a relatively modest part (in East Java about 17% of the cash value) of the home garden products, and most of these are the perennial types (Laumans et al., 1985). Also very important are the starchy tuber crops as a buffer for periods of scarcity of the main staples (mostly rice or maize). The leaves of cassava and sweet potato are important vegetables as well.
Another characteristic of the home garden is the very low input of capital (no agro chemicals, no special tools needed, planting material at hand) and the use of cheap family labour mostly in spare time. The soil is kept fertile by the household debris and manure. In this natural biological ecosystem with its great diversity of plants, the incidence of diseases and pests is generally very low (Soemarwoto, 1985; Landauer & Brazil, 1990).
Amidst all the praise for the home garden as a cropping system whose strength lies in stability rather than peak performance, it has become clear that home gardens are well suited to feed the family, but that commercial market gardens have to cater to the millions.
Extensive field production
The field production of vegetables for home consumption needs only a little space. It is common practice to use open places in the food crops and on field borders instead of planting entire plots with vegetables. Very common vegetables on dikes along rice fields are yard long bean, winged bean, lablab, kangkong, amaranth, caisin, eggplant, and pumpkin.
Numerous vegetables, each usually represented by a few plants only, can be found in the fields of food crops. Hence it is a form of mixed intercropping and constitutes the vegetable part of the subsistence farm system. Not much care is given to these vegetables. They do not get separate chemical spraying or fertilizers, but may profit from the treatment given to the main crop. The great diversity of species is the best guarantee for continuous production. About 50% of the vegetables for home consumption may come from this type of vegetable production.
A considerable part of the cash crop vegetables, possibly 20%, is also produced in a very extensive way, characterized by rain dependent production with low inputs of pesticides, fertilizers and labour and by low yield levels and poor quality with low prices. This explains the very low average yield level of many crops in national statistics, e.g. 3.2 t/ha for capsicum pepper in Indonesia. Upland fields used for the production of cash crop vegetables during the rainy season may also be planted with vegetables during the dry season, provided that sufficient irrigation water is available. Vegetables are also planted as dry season crops after rice, often on residual moisture. There is no abrupt border with the intensive production system described below; there is a gradation of intermediate cultural practices, from very extensive to very intensive.
Intensive market gardening
Intensive market gardening accounts for at least 65% of all vegetables marketed in South East Asia. Only a very small part (5%) of the harvested products is used for own consumption. The main features of this category are the high costs for labour and inputs (seed, fertilizers, pesticides), the professional application of cultural practices and the tendency to offer improved quality products for the organized marketing sector. Within this category of intensive gardening, a distinction can be made between:
- upland vegetable production. Rainfed or irrigated vegetable crops in permanent production or in rotation with other upland food crops, e.g. maize, soyabean, groundnut, or with sugar cane.
- wet field vegetable production. Vegetable production during the dry season, after the wet season paddy, is commonest, but permanent cultivation of vegetables also occurs. The advantage of growing vegetables after rice is that soilborne diseases are eliminated by the flooding of the fields; the disadvantage is the amount of labour needed for soil tillage.
Because many vegetables are short duration crops, they can often profitably be fitted into cropping systems based on food or industrial crops, in order to improve the cropping intensity of agricultural land. Therefore, a large proportion of the vegetables are not produced in sole cropping but in mixed intercropping. Which of these systems is chosen depends on many factors, e.g. the tradition of the farmer, the type of crops, the cost of labour, and potential for mechanization. In many cases, mixed intercropping takes the form of relay cropping, in which the growing period of a younger and an older crop, or of a long duration and a short life crop overlap. Farmers use mixed intercropping instead of sole cropping for economic reasons in order to:
- reduce the risk of losses. If one crop fails, the second or third crop growing at the same time in the same field may give a profit;
- make better use of the land. Young plants do not cover the land area completely and the sunlight is underutilized. Intercropping with other plants traps the available light as efficiently as possible. Relay cropping shortens the time in between harvests. A good example is capsicum pepper in Indonesia, planted on thousands of hectares in between shallots one month before the shallots are harvested.
- economize on production inputs (fertilizers, pesticides). Cabbage and capsicum pepper are planted between tomato and they profit from the pesticides and fertilizers applied to tomato.
Several other advantages from the agronomical and environmental point of view can be mentioned:
- pathogens, pest populations (thrips, aphids, mites etc.) and virus infections may be kept at a lower level, perhaps below the damage threshold. Tomato plants repel diamond back moth in cabbage. Maize plants give protection to the predators (natural enemies) of pests of capsicum pepper.
- the dense vegetation in mixed intercropping reduces soil erosion by heavy rainfall.
- minerals are better used and leaching is reduced.
- weeds are suppressed.
- bamboo or wooden poles for tomato or cucumber are used again by other climbing vegetables (loofah, bitter gourd, beans).
- the shade of the earlier crop (maize) is profitable for younger crops such as capsicum pepper.
But mixed intercropping certainly also has disadvantages:
- spraying with pesticides is no longer selective. Farmers use wide spectrum pesticides, they spray routine wise and do not respect the safety period needed before harvesting the earliest species in the mixture.
- crop rotation for reduction of soilborne diseases and pests is difficult. In Java a common relay cropping system lasting one year is tomato/cabbage (white or Chinese); harvested cabbage is replaced by capsicum pepper, French bean is planted against tomato sticks. In this system, the soilborne diseases club root, bacterial wilt and root knot nematodes will be maintained.
- manual and mechanized weed control are difficult.
Farmers know by experience which crops combine well. They combine plants with a certain tolerance of shade (capsicum pepper, welsh onion, Chinese cabbage) with tall crops (maize) or climbing species (leguminous vegetables, cucurbits). However, farmers have less knowledge of the crop rotation required to avoid soilborne diseases.
Some of the woody species in the vegetable assortment may have a role to play in agroforestry systems. This is a relevant possibility since an important issue for the future is how to combine agriculture and forestry in order to achieve sustained production of food, fuel and timber.
Although the use of improved commercial seed is rapidly increasing in South East Asia, a large part of the marketed vegetables is harvested from crops of local cultivars, landraces or farmers' selections. Self pollinated crops (e.g. tomato) breed true to type and the grower can easily obtain next season's sowing seed from a limited number of healthy plants. Maintaining the identity of cross pollinated crops (hot pepper, cucurbits) is more complicated. The grower has to remove plants that are off types in an early stage and he then has to take the seed from the best plants in the middle of his field. If homogeneity is really important for the grower then he could better leave the seed production to the professional seed growers.
The lack of modern selected cultivars and of an efficient seed supply system is a serious hindrance to the improvement of commercial vegetable cultivation. In most South East Asian countries a local seed industry is gradually emerging, and a start has been made to establish independent official control of the genetic integrity and physical quality of vegetable seeds. Imported seeds of European type vegetables are not usually adapted to tropical conditions and are often inadequately protected from the effects of high temperatures and humidity.
If the private sector is to invest in the development of improved cultivars, the crops concerned must cover sufficient area and it must be possible to cover the costs of development; seed firms therefore try to develop and market F1 hybrid cultivars, whose seed has to be renewed each season unless the lower yields through inbreeding depression in successive generations are accepted.
A great advantage of hybrid cultivars is that the period required to combine useful characters, e.g. for resistance to disease, is much shorter than in conventional cultivars. The creation of hybrid cultivars is relatively easy, especially in the case of solanaceous vegetables and cucurbits. Because the hybrid seed is so expensive, farmers often harvest the seed from their F1 hybrid crop for the next planting, but their experience with the segregating F2 hybrid material is in most cases very frustrating. The inbreeding depression is reflected in a lower yield and loss of uniformity and quality. In some cases this depression is relatively slight, e.g. for certain hybrids of onion, tomato, capsicum pepper. In watermelon the depression is so large that the product of the F2 is no longer marketable.
Modern seed companies strive to bring a complete assortment of vegetable seeds of the region into the seed merchants. This includes the many OPs, i.e. the open pollinated cultivars which might easily be reproduced by the farmers themselves. In practice, however, farmers gradually realize that it pays to buy all seed from a dealer, provided that the price is reasonable (Groot et al., 1988). The production of healthy and viable seed which is true-to-type is a skilled business.
The vegetatively propagated crops (garlic, shallot) are less attractive for seed companies because it is easy for the farmer himself to renew the expensive planting material. Yet for these crops too, professional seed producers will gradually take over from the farmers because they will supply superior virus free planting material. For vegetatively propagated vegetables like aibika, the kangkong type reproduced by cuttings and star gooseberry, farmers must rely on their own planting material.
Table 5 gives some data on the seed of commercial vegetables.
In the future crop husbandry will have to concentrate on the efficient use of resources and approaches to recycling. Better agronomic practices can reduce soil erosion and lower chemical inputs. Because of the high value of vegetable crops and their adaptability to different cropping systems, manuring and recycling of plant nutrients can be promoted.
Fertilizer recommendations can be arrived at in two ways:
- Soil analyses combined with field trials result in response curves for macro nutrients. If the farmer has soil samples analysed before planting, these curves can be used to translate soil analysis data into precise recommendations for his own field, but this seldom occurs. Normally, these data are used by agronomists and extensionists to prepare general average recommendations for the farmers, taking the soil and crop type into consideration but not the condition of the specific field. Table 6 gives a valuation of soil analysis data.
- Plant analyses and yield data give information about the amounts of nutrients taken up by the crop. The best indication gives the analysis of the total biomass of the harvested plant parts (removal of nutrients) and the plant parts remaining in the field (temporarily immobilized nutrients), the total being the nutrient uptake of the crop. Some examples of the uptake of macro nutrients are presented in Table 7.
Assuming that the non harvested plant parts will remain in the field, it still must be realized that the uptake or absorption, and thus immobilization, of minerals is much greater than the actual removal from the field in the harvested part. The uptake of minerals seems to vary greatly between species, but there is much conformity for crops within the same species. Variations are caused by growing conditions, varietal differences and soil properties. A higher than normal uptake (luxury consumption) is caused by too high a supply, e.g. of N or K.
In practice, the yield expected from a good crop under local conditions is often taken as criterion for a calculation of the fertilizer recommendation. When estimating the adequate fertilizer gift based upon the amounts taken up, losses by leaching or immobilization have to be compensated for. For example, it has been found that the uptake of a shallot crop producing 15 t/ha of bulbs in Indonesia was 80 kg N, 15 kg P (35 kg P2O5) and 90 kg K (105 kg K2O). The recovery (utilization rate, efficiency factor) was estimated at respectively 60%, 40% and 70% of the nitrogen, phosphate and potassium fertilizer, leading to a recommendation of 80/0.6 = 130 kg N, 35/0.4 = 90 kg P2O5 and 105/0.7 = 150 kg K2O.
Usually, skilled vegetable farmers apply manure, compost or other organic fertilizer whenever it is available, in quantities from 10-30 t/ha or more. Apart from improving the physical properties, this manure will amend the soil with considerable amounts of nutrients. For instance, 10 t of cow dung (LEHRI, West Java) contains 260 kg N, 45 kg P (corresponding to 100 kg P2O5) and 130 kg K (corresponding to 160 kg K2O). These minerals are partly fixed in the organic material and are therefore released gradually. Thus, in the shallot example mentioned above, 10 t of cow dung might cover the entire uptake of these macro nutrients. Unfortunately, in shallot production areas (Brebes Tegal), insufficient farm manure is available and all nutrients are applied as mineral fertilizer.
The plant needs different quantities of nutrients during its lifetime. In order to reduce losses by leaching, especially in the rainy season, it is good practice to supply the N and K fertilizer, or at least the nitrogen, in split applications. The phosphate should preferably be given during ploughing or tillage, together with the organic manure, because it is not leached.
Many vegetable growers use foliar sprays of mineral fertilizers, often mixed with pesticides in the same sprayer. The quantity of macro nutrients which can be applied with this method is very limited. The most profitable is the application of urea: with a 5% solution and 500 l/ha, the rate is 25 kg/ha of urea or only 12 kg N. Many types of foliar sprays with N, P, K, Mg and micronutrients are on sale. Apart from being expensive, they are superfluous in normal growing conditions. The drawbacks of foliar application are the risk of scorching the plant, the possible interference with the action of pesticides, and the corrosion of the sprayer apparatus. But foliar application may be justified to cure an apparent deficiency of a micro nutrient, e.g. borium or iron deficiency on alkaline soils.
In practice, many types of organic waste material and all types of manure including nightsoil are used for vegetable production. If the C/N ratio of the material is above 15, as in the case of rice straw and bran, an addition of N fertilizer (7 kg per t straw) is recommended to avoid N deficiency. The high soil temperature in the lowlands ensures organic material decays fast. Crops such as amaranth can be grown successfully on fresh or only partly decomposed town waste, although there is a risk that this waste will pollute the soil with plastics and heavy metals.
Another way of increasing the organic matter content in soil is to grow a cover crop which is ploughed in before planting the main vegetable crop. A leguminous plant (e.g. Crotalaria) which fixes nitrogen through Rhizobium bacteria is normally used for this green manure. Although highly recommended by researchers, green manure is infrequently used by vegetable growers, for economic reasons. Mulching of vegetables with straw, usually rice straw, is a very common practice among vegetable growers in South East Asia. Apart from reducing the growth of weeds, it limits sun burning of the organic material, impedes soil erosion, and keeps the soil cool and moist. The straw mulch gradually decays and becomes available to the soil as organic manure.
Vegetables in general are succulent crops and attractive to pests and disease organisms. In the international terminology, the word "pest" is interpreted in two senses. In the broad sense it means any organism that hampers the crop: weeds, insects, mites, snails and slugs, rodents, birds, nematodes, fungi, bacteria, viruses. The word "pest" in Integrated Pest Management (IPM) fits in this concept. In the more usual terminology all animal causes of plant damage except nematodes are called "pests", whereas the microorganisms including nematodes are grouped as "diseases" and the noxious plants competing with the crop are referred to as "weeds".
Exact information on the economic level of crop losses is limited. It is difficult to assess the losses caused by a single pest or disease. Crop health is the complicated outcome of the attack by several organisms trying to proliferate on plants with a genetically determined constitution, which is strongly influenced by the ecology. Overall yield losses in the vegetable sector may amount to 25%, which is higher than for all other categories of crops. For the farmer, the costs incurred for the chemical control of pests and diseases are very high, often between 100 and 400 US$ per ha or 10-40% of the variable costs (material inputs and labour). Diseases and pests cause a downgrading of the market quality and consequently of the farm gate prices, and reduce the export chances.
Crop protection has evolved along with the crops and the cropping systems. In home gardens, fences were constructed to protect the vegetables from larger animals. Other simple control measures were the manual removal of caterpillars or repelling the insects with wood ash (a kind of chemical control). With the production of vegetables for the market, pest control measures became more urgent, in order to achieve the highest possible yield of products undamaged by pests or diseases. A number of non indigenous pests such as the diamond back moth (Plutella xylostella) and club root (Plasmodiophora) on cruciferous vegetables have been introduced into South East Asia with planting material or otherwise and have become extremely troublesome (Eveleens & Vermeulen, 1976). Compared with temperate countries, little is known of diseases and pests of vegetables in the tropics. The diseases and pests of individual vegetable crops are mentioned in the species descriptions. This section is restricted to some general observations about their control.
Although very costly, the application of pesticides has become the most common and easiest way of pest control in vegetables. Although there is a growing awareness of the dangers of toxic substances, the use of these biocides is still increasing and has reached levels at which there are health risks to growers and consumers, and considerable damage to the environment. The preventive spraying of pesticides has become routine, especially on the commercial highland vegetables of foreign origin, which lack the internal defence mechanisms of indigenous vegetables.
Encouraged by a sometimes rather aggressive sales promotion by chemical companies and by ineffective governmental control on toxicity or residual effects and by the lack of know how among farmers and extensionists, the use of pesticides often leads to the intoxication of the people handling the pesticides and to noxious effects for the health of the consumers. In many places in South East Asia the environment, the land and the water for drinking or fishing, has become polluted.
Chemical control has a serious negative side effect; it destroys predators, parasites or natural enemies of the pest. This disturbance of the natural balance leads to a further intensification of the chemical treatments. Many pests have developed resistance to pesticides, forcing the farmers to spray more frequently and with stronger concentrations. The control of the diamond back moth on cabbage is a notorious example. Yet another negative effect which is rarely recognized by the farmer is that many pesticides are phytotoxic. Crop damage often occurs when pesticides are sprayed in higher concentrations than prescribed, especially during the dry season. The concentration of fungicides on the leaves of e.g. tomato and capsicum pepper is often so high that the stomata are blocked and photosynthesis is hampered.
Thus, chemical treatments should be applied only when the economic threshold for damage is surpassed, when no other control measures are effective and when precautions are taken for safe use.
The use of natural enemies to control a pest, i.e. predators, parasites or diseases, is called biological control. Many predators of insect pests on vegetables have already been found in South East Asia. The rearing and release of egg parasitoids of the genus Trichogramma and larval parasitoids of the genus Diadegma for the control of diamond back moth (Plutella xylostella) on cabbage has already been tried out in the highlands of the Philippines and Malaysia, apparently with some success (Talekar, 1992). The spraying of a bio insecticide produced by the bacteria Bacillus thuringiensis, often called BT, is a promising microbial control method. Several strains of BT are known. This microbial insecticide is sometimes used against caterpillars in vegetable crops. The disadvantages of BT are that it is rather costly and that the insect population gradually develops resistance.
Control with cultural practices
Depending on the type of pest, the crop and the environment, the damage caused by pests can be kept at a low level with the right cultural practices. Whether these cultural practices are economic depends strongly on the local conditions and the skill of the farmer. Several cultural practices are known to reduce pest incidence and damage. "Crop rotation" is effective in the control of soilborne diseases and sometimes also against insect pests. With the right "time of planting", also called "timing or planning of crop production", certain important pests may be avoided; insects are often more abundant in the dry season, whereas fungal diseases are worse during the rainy season. "Mixed intercropping" sometimes reduces the pest incidence, e.g. tomato plants and garlic are known to repel insect pests of cabbage or carrot. "Disinfection by heating" of nursery soil against pathogens like Pythium (damping off) is sometimes practised, e.g. for raising tomato or capsicum pepper transplants. A "balanced fertilization" with a reduction of the often too high dose of nitrogen makes the crop stronger and less attractive to pathogens. The "soil structure and pH" are very important for crop health. Organic manure improves the soil structure and reduces bacterial wilt and nematodes. "Liming" of acid soils reduces club root disease of crucifers. Good "drainage" reduces bacterial wilt and fungus diseases of many crops. "Mulching" with rice straw or plastic is a method of weed control and reduces soil erosion and bacterial wilt. Plastic mulch is reported to reduce thrips and aphid populations. In some areas, farmers raise nursery plants (cabbage, capsicum pepper, tomato) and even whole crops (cabbage) under fine mesh insect proof "nylon netting". Netting whole fields of Chinese kale (kailan) is used against diamond back moth in Thailand. Good "sanitation" is another helpful practice. This means the removal of crop residues and of infected plants or planting material (roguing). In shallot growing in Indonesia it is common practice to pick off all Spodoptera caterpillars and egg clusters by hand and to destroy them.
Control with resistant cultivars
The cheapest and most practical control method is to use resistant cultivars. Landraces generally possess high "horizontal resistance", a genetically determined level of tolerance, which means the plants are attacked but do not suffer very much. This is in contrast to many resistances in modern cultivars, which are narrowly based on one or a few genes. These resistances are often broken in a short time, by the pathogen evolving and forming new strains or races. Plant breeders have developed hundreds of cultivars of the more important commercial vegetables with resistant genes against fungal or bacterial diseases, nematodes and viruses, but resistance to insects or mites is very rare. The existence of resistant cultivars is mentioned in the species treatments.
Integrated Pest Management (IPM) is a worldwide accepted control method for diseases and pests. It is a combination of non chemical control measures (resistant cultivars, cultural practices, biological control) with a minimum use of indispensable pesticides based upon threshhold observations. It is mostly practised for insect pests and often concentrates on a single major pest such as diamond back moth of cabbage.
If the overall health condition of the crop is taken as the major issue, a more holistic approach to integrated control is the Integrated Crop Management (ICM), which takes the coherence and relationship between human and environmental factors into consideration. ICM is defined as "a system whereby all interacting crop production and pest control tactics aimed at maintaining and protecting plant health are harmonized in the appropriate sequence to achieve optimum crop yield and quality and maximum net profit, in addition to stability in the agro ecosystem, benefiting society and mankind" (El Zik & Frisbie, 1985).
Harvesting and post harvest handling
First of all it is important to realize that pre harvest choices such as cultivar and cultural practices, strongly influence the quality obtained at harvesting. Size, form, colour, firmness, taste and other internal and external product qualities are genetically determined. During cultivation, all measures which assure good health of the crop also have an impact on the post harvest quality. The plant density influences the product size and form. A common mistake is to apply too much nitrogen fertilizer, which makes the harvested product more watery, weaker and more susceptible to damage and rotting.
Most vegetables are very perishable products. The losses of product and deterioration of quality caused by inappropriate harvesting and post harvest handling are considerable. Losses of one third of the harvested product are not exceptional. Harvested vegetables are still living parts of plants, which remain very susceptible to damage until ultimate consumption. Vegetables have a high water content (70-95%) and the leafy types in particular will wilt easily because of continuing respiration after the harvest. Some recommendations regarding the correct handling of harvested vegetables to minimize losses are given below.
Two types of harvesting methods may be distinguished. Once over harvesting is the harvest of all the useful parts or of all plants at once. This is practised e.g. on carrot, radish, cabbage, onion, garlic. More common is the repeated harvesting of the plant parts desired in several picking rounds, e.g. for capsicum pepper, cucumber, tomato, yard long bean, asparagus. In many cases the grower himself can choose which type of harvest will be applied. Many leafy vegetables (amaranth, kangkong) can be once over harvested by uprooting or cutting the whole plants, or they can be harvested repeatedly by successive cuts. In the latter case, the cultural practices will be different; in the example of amaranth, the amount of nitrogen fertilizer, the height and frequency of cutting, the plant spacing and their interactions will strongly influence the ultimate yield and quality, in particular through their effect on flowering (Grubben, 1976).
The maturity stage of the product wanted is greatly influenced by the time and frequency of harvesting. For example in tomato, the farmer has to consider the maturity stage requested by the dealer. If harvested immature green, the tomatoes will not taste good. Mature fruits have the best taste but will not tolerate several days of transport and storage. The farmer will try to compromise by harvesting at the mature green stage to let the fruits ripen in transit or storage before marketing. To deliver high quality products, the farmer must have a good knowledge of harvest indices: size, colour, firmness.
Post harvest handling has the objective of bringing the harvested product to the consumer with a minimum of quality deterioration. A first step is a sorting into various quality classes or gradings. The principle is a two way sorting, i.e. by appearance and size. An example is presented in Table 8, giving the prescriptions for the eight quality classes used in the trade of shallot in Indonesia (Schoneveld, 1992).
Packing and transport
Proper packaging is aimed at avoiding mechanical damage by pressure and at avoiding warming up from respiration by inadequate ventilation. Excessive ventilation is also undesirable, as it results in wilting and weight loss. The packing materials used for vegetables are very diverse: net bags, bamboo baskets, wooden or plastic crates, cardboard boxes and plastic bags, and also loose on the truck. The choice is purely economic: cheap packing materials generally lead to more deterioration of quality and a lower price. A suitable packing unit is 20 kg. Bags with dried products such as garlic and shallot may contain 40 kg. Packing units are often made too large and too heavy (sometimes 100 kg baskets or bags) for easy handling. They lack sufficient ventilation, and heating of the product can easily cause serious rotting. The main cause of post harvest losses during storage and transport, however, is the pressure from the product loaded on top. Figure 2 illustrates how the damage by pressure can be reduced by the installation of partition floors or by using self supporting crates (Schoneveld, 1992). Clearly, the pressure on the lowest product is least for solution G (small self supporting crates).
Vegetables can be stored in a cool, dark, well ventilated place. Leaf vegetables must be wetted occasionally, to avoid drying out. The best keeping is in a cool room, but this method is too expensive for the individual farmer and for most dealers. Some vegetables keep well at low temperatures of 1-2°C (Allium crops, cabbage, radish), but most other products will suffer damage at those temperatures. For example capsicum pepper stores better at 5-7°C, cucumber at 10-12°C. Onion, shallot and garlic also store well in the lowlands in well ventilated sheds; a temperature above 27°C impedes early sprouting. Leafy vegetables may be packed with shredded ice to keep them fresh during long distance transport. Fresh exports require sophisticated post harvest facilities and transport infrastructures to deliver fast and timely, as well as top quality produce.
Utilization and processing
The utilization of vegetable products is changing constantly. The development of new types of food is in general leading to higher levels of consumption. In South East Asia it is customary to consume vegetables as fresh as possible. Many housewives buy fresh vegetables once or even twice a day. Yet processed vegetables are becoming more popular for reasons of convenience.
Processing techniques are of the utmost importance in the vegetable sector, because of the perishable nature of the product. Apart from adding value, processing enables the fresh market to be relieved when prices are low due to a glut in production, and also avoids wasting produce which is not marketable because of its small size or less attractive appearance.
Vegetable products are mainly processed by drying or dehydration, pickling, canning, and freezing. Drying or dehydration is one of the oldest preserving methods; the principle consists of reducing the moisture content below that at which microorganisms grow and reproduce. It is usually accomplished through heat (e.g. sunshine) and ventilation; for aromatic vegetables dehumidifiers are more suitable, in order not to lose the volatile oils. The drying of green leaves (Corchorus olitorius L., Sesamum radiatum Thonn. ex Hornem.) and fruits (okra, capsicum pepper, local eggplant, pumpkin) and their preservation as powder is common practice in Africa. Some of the food value is lost in the process, but drying merits more investigation as it is a simple technique that can be widely used throughout the tropics. Pickling is preservation in brine or vinegar, with or without bacterial fermentation. There are many traditional methods for preparing salted and pickled vegetables in South East Asia.
Mixed vegetable and fruit juices are becoming increasingly popular.
Canning fruits and vegetables is becoming an established practice in South East Asia, but preservation by freezing is still in its infancy.
Genetic resources and breeding
Knowledge of the use of wild plants is disappearing rapidly. It is only natural that the importance of the wild flora as a direct food source is decreasing, but it is inadmissible for genetic resources to be destroyed before the true value has been assessed objectively.
The development of modern horticulture has led to a huge reduction in the number of vegetable species. It has also resulted in a narrowing of the genetic base of the remaining species, because a large number of local, unselected cultivars have been replaced by fewer highly selected planting materials. Plant breeders have to rely on genetic resources, which can be found in the primary and secondary centres of diversity or in artificial germplasm collections.
The establishment of the International Board of Plant Genetic Resources (IBPGR) in 1974 greatly increased the awareness of the importance of crop germplasm and led to the establishment of numerous collections of species endangered by genetic erosion. Based on a study of tropical vegetables and their genetic resources (Grubben, 1977), in 1979 IBPGR prioritized eight vegetable genera or groups for immediate action, i.e. Abelmoschus, Allium, Amaranthus, Capsicum, Cruciferae, Cucurbitaceae, Lycopersicon, and Solanum melongena (van Sloten, 1980).
Information on existing collections was compiled in a Directory of Germplasm Collections. 4. Vegetables (Bettencourt & Konopka, 1990).
Selection and breeding have an important role to play in the improvement of vegetable crops. Named, well defined cultivars have so far only been developed in an estimated 60 species out of the 225 primary use vegetables described in this book, and a large part of these originate from outside South East Asia. The development and release of cultivars in most South East Asian countries is still the task of public research and extension agencies, although the involvement of the private sector is increasing.
The establishment of the Asian Vegetable Research and Development Center (AVRDC) in 1971 has given a strong impetus to the development of advanced breeding programmes in South East Asia on a number of vegetable crops such as tomato and Chinese cabbage. Its training programmes have also strengthened the national programmes in other crops.
There are two aspects to the breeding philosophy in South East Asia. The first, easily overlooked or neglected, is to select or breed for low input farms where standard cultivars are required that respond to low levels of fertilizer, are adapted to a wide range of environmental conditions, and are tolerant of common diseases and pests. This implies collecting, evaluating and maintaining the germplasm of a wide range of crops, doing research and gathering information on crop characteristics, and finally, selecting suitable cultivars by conventional selection techniques. These tasks are best performed by government research and extension agencies with an overall responsibility for the sector; they should supply local private seed companies with breeding material or selections to be multiplied into commercial, good quality seed for the farmer.
The other aspect is the development of high yielding cultivars for commercial farms, which give maximum response to optimal input. It is at this level that good commercial opportunities exist for the private sector.
This volume is proof of the great wealth and diversity of vegetables in South East Asia. The development of the horticultural sector is first and foremost a matter of allocation of resources. However, new revolutionary solutions have to be found to achieve sustainable production systems. As the train of horticultural development gathers speed, efforts should be made not to repeat the mistakes of the industrialized world, i.e. environment-unfriendly production methods and a considerable loss of genetic diversity.
Notwithstanding the significance of the vegetable sector in the agricultural economy, the diffuse distribution and species diversity have made it hard to develop a compelling rationale for allocating appropriate resources for vegetable crop research.
Most countries in South East Asia have facilities for vegetable research (LEHRI, Indonesia; MARDI, Malaysia; IPB, the Philippines; Institute of Horticulture, Thailand; Institute of Agricultural Science, Vietnam), but in general the allocation of resources does not reflect the economic (and nutritional) importance of the sector. In setting priorities within the sector, the "exotic" highland species (cabbage, potato, etc.) have received much attention, and the indigenous vegetables have scored low, but they have not yet lost the battle. Numerous new initiatives have recently been taken to promote lowland vegetable research, e.g. at LEHRI, Indonesia.
At the international level, the Asian Vegetable Research and Development Center (AVRDC) has done pioneering research on Chinese cabbage and tomato. In its strategic plan for the 1990s (AVDRC, 1991), it has clearly opted to give first priority to the lowland humid and subhumid tropics, to concentrate on small scale commercial production, and to expand its commodity coverage to capsicum peppers, eggplant, and the important Allium crops (onion, shallot, garlic). It plans to put more emphasis on a decentralized organizational set up with regional research networks, and by so doing is pursuing the same line as the Consultative Group on International Agricultural Research (CGIAR), which is studying new ways of promoting tropical vegetable research, possibly through a new coordinating body (such as IBPGR) with the task of stimulating the development of national research systems (Winrock International, 1986).
National policies tend to emphasize the development of exports rather than domestic consumption, but a well supported domestic market is the best possible basis for export. The rapid expansion of the supermarket system of selling fresh vegetables, with its insistence on quality, will in time stimulate the adoption of improved marketing methods. The prospects for fresh exports to large urban centres (Singapore) and nearby industrialized countries (Taiwan, Japan) are certainly good, but more is to be expected from exports of preserved and processed products. However, this export sector should develop as a by product of processing industries aimed at the large domestic markets. Vegetables in general have a positive income elasticity and with increasing economic prosperity, the production of market vegetables will increase.
The rapid expansion of commercial vegetable production creates a market for high quality seed. Growers are changing their attitude from considering vegetable seeds as a cheap internal input to the conviction that it pays to start a crop with healthy market seed of an improved cultivar purchased from a professional seed producer.
In some cases it may be justified that the public sector (National Agricultural Research Systems) produces market seed itself. However, international experience has proven that farmers are generally better off when the public sector takes care of the more fundamental part of research in support of the private seed sector. The public sector should be responsible for independent testing of the value of new and existing cultivars, the release policy for new cultivars, and the control of seed quality.
The size of the national seed market determines whether the seed can be produced in a country. A sound government policy should stimulate breeding activities and seed production in the vegetable production areas of the country in the interest of farmers and consumers. Apart from a few exceptions (e.g. white cabbage), it is technically and economically feasible to produce all vegetable seed in the South East Asian region.
G.J.H. Grubben, J.S. Siemonsma & Kasem Piluek