Fragaria ananassa (PROSEA)
Fragaria ananassa (Duchesne) Guédès
- Protologue: Taxon 33: 724 (1984).
- Family: Rosaceae
- Chromosome number: 2n= 8x = 56
- Fragaria ananassa Duchesne or Fragaria vesca L. race ananassa Duchesne (1766),
- Fragaria chiloensis (L.) P. Miller × Fragaria virginiana P. Miller (± 1750).
- Strawberry, dessert strawberry, garden strawberry (En)
- Fraise, fraisier ananas (Fr)
- Indonesia: arbèn
- Malaysia: strawberry
- Philippines: fresa
- Thailand: stroboeri
- Vietnam: dâu tây.
Origin and geographic distribution
F. ×ananassa is derived from natural crosses between the American octoploids F. chiloensis and F. virginiana in Europe around 1750. Modern cultivars are derived from further crossing and selection among the hybrids and - sometimes - backcrossing with F. chiloensis .
The cultivated strawberry is grown extensively in most temperate and in some subtropical countries. In the tropics strawberries are grown in the highlands. In northern Thailand strawberries are grown commercially by many growers, be it on a small scale. A few larger producers are found in the Dieng Plateau in Java and near Berastagi in Sumatra. Elsewhere in South-East Asia the fruit is grown and marketed more incidentally.
Only the ripe fruit is used. While much is consumed fresh, most is preserved, mainly by pulping in sulphite or deep-freezing. These preserves are raw materials for strawberry products such as jams and tarts or dairy products containing strawberries - ice cream, yoghurt, custard. Deep-frozen fruit can be reconstituted whole; this is also possible for fruit that has been dehydrated by reverse osmosis in a sugar solution, followed by air drying.
Production and international trade
According to FAO statistics world strawberry production increased from 1.8 million t per year during 1979-1981 to 2.27 million t in 1988. About half the crop comes from Europe (mainly Poland, Italy and Spain) and one-quarter from the United States. Production in the tropics is insignificant and only meant for the domestic market, but this includes, for instance, the transport of graded and carefully packed fruit by air from Chiang Mai to Bangkok and from Sumatra to Singapore.
In spite of the perishable nature of the fruit there is considerable international trade, mainly to extend the season of fresh supplies. However, the quantities involved are small compared with the trade in processed strawberries.
Thai sources give the following composition per 100 g edible portion: water 90.6 g, protein 0.8 g, fat 0.5 g, carbohydrates 7.6 g, fibre 1.7 g. Vitamin content is low, but vitamin C (53 mg/100 g) is about average for fruit. The energy value per 100 g is 140 kJ.
- Perennial herb with short stem, rosette of leaves and runners.
- Leaves trifoliolate; petiole 1.5-17 cm long, soft hairy; leaflets 1.8-7 cm × 1.3-6 cm, with short petiolules or almost sessile, margins serrate, glaucous at lower surface.
- Inflorescences erect, up to 26 cm long, up to 16 flowers in a dichasium; peduncle 0-14 cm, pedicels up to 9 cm long; flowers functionally unisexual or bisexual, 5-6-merous; floral cup 4-6 mm diameter; epicalyx leaves 5-8 mm × 2.5-3.5 mm; sepals 7-12 mm × 3-4.5 mm; petals suborbicular, 9-12 mm wide, shortly clawed, white; stamens 25-37, sterile in female flowers; disk a thickened, 5-6-lobed ring; pistils many, in male flowers ovaries not developing.
- Pseudocarps (false fruits) globose, obovoid or ovoid, up to 4.5 cm × 5.5 cm.
- Achenes (true fruits) sunken in the swollen torus, 1.25-1.5 mm × 1-1.25 mm, smooth.
Growth and development
Strawberry plants are formed on runners, prostrate stems emerging from the leaf axils of mature plants. After forming 2 internodes the runner terminates in a leaf rosette which becomes an independent plant by striking roots adventitiously; the runner resumes extension growth through the axillary bud in the last internode, again forming a terminal rosette after 2 internodes, etc. Each new rosette produces a series of leaves until the apex turns floral, after which the bud in the upper leaf axil breaks to extend the rosette. Thus the growth habit of runners and rosettes is sympodial and this also applies to the inflorescences: the axis terminates in a flower and secondary axes emerge from the subtending node.
Axillary buds may give rise to runners, lateral rosettes (additional "crowns" which correspond to runners but lack the extended internodes), or inflorescences. When growing conditions are favourable crowns are formed, but when gibberellin levels are high (e.g. in plants exposed to winter cold) runner formation takes precedence; formation of inflorescences requires short daylengths.
In the tropics where days are always short, flowers can be initiated throughout the year from the moment the plants have reached a certain minimum size. In the absence of winter cold few if any runners are formed, but as growing conditions are favourable, there are many crowns per plant. Whereas this is the general pattern in the tropics, it is not always clear how the fate of the axillary buds is determined. Moreover, cultivars differ greatly with respect to critical daylength - some are even day-neutral - and chilling requirements.
In northern Thailand at a latitude of 18°N, "Tioga" follows a clear seasonal pattern of growth: runners form throughout the rainy season from May to September; thereafter crowns form; flowering starts in October and fruit can be harvested from November through April or longer, depending on the onset of the rains.
Both wind and insects - especially honeybees - appear to play a role in pollination and nearly all pistils in a flower must be pollinated to obtain a full-sized, regular shaped fruit. Flowering continues for one month or more at high latitudes, the sympodial habit of the inflorescence leading to a slow progression of flower opening. The fruit ripens within one month after flowering, fruit size declining for the higher-order ramifications of the inflorescence. Although continuous flowering and fruiting is possible in the tropics, the crop in the rainy season is often not worthwhile because of a low growth rate, poor pollination and high incidence of fruit rot.
Other botanical information
The best-known wild strawberry is Fragaria vesca L., a diploid species that is often grown in home gardens - also in various highland areas in Malesia. The fruit has the true strawberry flavour; to transfer this flavour to F. ×ananassa octoploidy has been induced in F. vesca so that both species can be crossed.
Numerous strawberry cultivars exist, including day-neutral, perpetual-bearing cultivars. Cultivars bred at high latitudes are ill-adapted to lower latitudes; in the tropics cultivars bred in California have generally done best. "Tioga", which in California has long been superseded by new cultivars ("Pajaro", "Chandler" and "Selva", a day-neutral cultivar), is still the leading tropical strawberry; the prolonged season under continuous short days does not exhaust the plants, which produce a high yield of firm fruit that travels well. Moreover, "Tioga" does not degenerate as rapidly as other cultivars under the pressure of virus and nematode infections which build up as a result of continuous vegetative propagation.
Strawberries thrive in mild climates, without extremes of temperature and humidity. Desiccating conditions are not tolerated; hence the importance of windbreaks, mulching and irrigation to limit stress. Resting plants survive -15 to -20°C, but flowers and young fruits are blackened by -1 to -3°C. At high latitudes the season is extended by growing strawberries in glasshouses or in polythene tunnels. Short daylength is needed for the initiation of flowers and a cold-induced rest period stimulates runner formation after growth resumes. Flowering and fruiting demand a fairly dry season; wet conditions interfere with pollination and control of fungi. In the tropics cultivation is successful at elevations above 1000 m.
Strawberries require a well-drained soil with a good waterholding capacity in the topsoil; in waterlogged conditions soil fungi kill the plants.
Propagation and planting
Runners are the natural mode of propagation. Where not enough runners are produced, plants are split into separate crowns, but this gives a multiplication rate in the order of only 5:1. Moreover, it is undesirable to use plants from production fields for propagation, since these fields cannot be adequately protected against virus and nematode infections. Propagation through tissue culture is possible and may be a very attractive alternative to the use of crowns.
Runners should not be taken from production fields either, but be grown from certified motherplants in fields at a safe distance from the areas of production. For the tropics it is advisable to import clean stock every few years. If this is done after the plants have been exposed to cold, they can first be used to produce runners, so that a small import consignment suffices. Propagation fields need to be protected against virus vectors and should be thoroughly weeded; geese have been used successfully to control weeds. Runners can be kept in cold storage at -2°C to be planted at any time of the year.
"Tioga" in northern Thailand freely forms runners during the rainy season and by the end of August, 30 plants may be gathered from a single motherplant, which is an exceptionally high number. The preferred planting time in the tropics is the rainy season; low night temperatures also help to limit stress after planting. Plants are spaced in double rows on raised beds, at approximately (70 + 30) cm × 25 cm, giving a density of 8 plants/m2. Spacing depends greatly on the intended duration of cropping; if plants are cropped for several years, they grow so much bigger that the density may be halved and single-row planting (100 cm × 25 cm) is practised.
Correct planting depth - the centre of the crown being just above the ground - and assured moisture supply are vital for a good start. When splitting plants, the bottom of the crown should be trimmed, leaving only the part of the crown with white roots.
Strawberries like a high organic matter content and the beds should be generously manured before planting. The crop responds well to nutrients, but if growth is too vigorous, flowering suffers and the dense foliage greatly complicates crop protection. Several top dressings with balanced fertilizer are applied, if necessary supplemented by liquid feed. In Thailand NPK 13-13-21 is considered best for the loamy soils. Mineral deficiencies of minor nutrients are widespread.
The beds or ridges are mulched with straw; in Thailand the large leaves of "hiang" (Dipterocarpus sp.) are strung together along strips of bamboo to neatly cover the sides of the ridge. Polythene may also be used; it is laid down before planting and pierced at each plant position. Mulches keep the fruit clean, conserve moisture and suppress weeds. Strawberries are very susceptible to weed competition. The raised beds facilitate furrow irrigation, but water and nutrients cannot easily reach the roots. Seep hoses can be laid on top of the beds - if need be, even under the plastic mulch - and connected to siphons in the feeder ditch for more efficient surface irrigation.
Young plants sometimes have to be deblossomed till they are sufficiently vigorous to bear. If the foliage becomes very dense, it may be mown off. Mowing is often needed in fields which are kept for 2 or even 3 years; in the tropics a crop duration of one year is more common. Narrow crop rotations should be avoided; in Thailand it is recommended to grow mung bean before planting strawberries.
Diseases and pests
Strawberries suffer from a host of diseases and pests. Diseases include several soilborne fungi such as Verticillium wilt and Phytophthora fragariae , red core. Crop rotation and proper soil aeration are the main control measures. Leaf spot (Mycosphaerella fragariae) may be the most widespread foliage disease, but leaf scorch (Diplocarpon earlianum) is more destructive. Powdery mildew (Sphaerotheca alchemillae) is common, but "Tioga" is not very susceptible. Anthracnose (Colletotrichum fragariae) affects leaves, runners and inflorescences. Grey mould (Botrytis cinerea) infects the flowers and remains latent until the fruit ripens. Partial control is achieved by preventing flowering in the rainy season; fungicides are most effective if applied during blossoming.
Virus diseases such as yellow edge and crinkle are spread by infected runners, and so are leaf, stem and root nematodes; aphids act as vectors for the viruses. In due course these infestations build up and lead to degeneration of a cultivar. Tissue culture makes it possible to make a clean start again, but usually new cultivars supersede the degenerating ones. In any case, every effort should be made to raise healthy runners.
Aphids, mites, root weevils and grubs, fruit worms and slugs are important pests. Control is complicated because potent pesticides cannot be used during the long harvest season.
Strawberries are picked daily and only in the early morning before they have accumulated much field heat. Ideally the tender fruit should only be touched once, which means that picking, grading and packing are accomplished in a single move. Fruit for the fresh market is held and detached at the stalk; the picker uses a tray filled with punnets and grades and packs by putting each fruit in the proper punnet. This only works if there are no more than 3 grades, e.g. large, small and reject. Rejects should also be picked because rot spreads quickly from one fruit to another. To keep the tray clean it rests on a wire frame that keeps it off the ground.
In Thailand the firm fruit of "Tioga" is picked rather early - 30% or 60% pink colour - and sized in 4 grades in the packing shed.
The Californian cultivars stand out because of their long harvest season and astounding yields. Mean yields in California in the 1980s ranged from 50-60 t/ha per year, against 20 t in Florida and less than 10 t/ha in the more northerly states of the United States. Reports of yields of 25-40 t/ha per year from Africa suggest that Californian yield levels might be attained in the tropics. Average yields of 12.5 t for the lowlands and 17.5 t for the highlands of northern Thailand also indicate that strawberry is a heavy cropper in the tropics.
Handling after harvest
At the packing shed, punnets are rearranged to fill each tray with a single grade. Punnets are covered with cellophane which remains free of condensation and hence clear to show the attractive contents. The fruit should not be allowed to warm up from the moment it is picked until it is sold. Even so the produce must be consumed within 2-5 days after harvest.
There are only very limited cultivar collections in South-East Asia. Since "Tioga" and other Californian cultivars are so much better than cultivars from higher latitudes, it makes sense to introduce and test the new cultivars released in California.
There is much domestic demand for strawberries in South- East Asia. Propagation and husbandry differ greatly in different production centres and much could be gained from exchange of experience. The high yields that can be attained, the long season, and the labour-intensive nature of the crop suggest that there may be scope for producing strawberries for processing in the tropics. In South-East Asia the potential is best for the drier highland areas.
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Choopong Sukumalanandana & E.W.M. Verheij