Changes

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.

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).

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).

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).

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.

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.

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.

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.

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.

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.