Allium cepa Aggregatum (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Allium cepa L. cv. group Aggregatum

Protologue: Sp. pl.: 300 (1753). Cv. group Aggregatum: Jones & Mann. Onions and their allies. p. 31 (1963).

Family: Liliaceae

Chromosome number: 2n= 16

Synonyms

• Shallot: Allium ascalonicum auct. non Strand, A. cepa L. var. ascalonicum Backer (1951).

• Potato onion: Allium cepa L. var. aggregatum G. Don (1827), A. cepa L. var. solanina Alef. (1866).

Vernacular names

• Shallot, potato onion, multiplier onion (En)
• Echalote, oignon patate (Fr)
• Indonesia: bawang merah, brambang (Javanese), bawang beureum (Sundanese)
• Malaysia: bawang merah, bawang kecil
• Papua New Guinea: lip anian
• Philippines: sibuyas tagalog (Tagalog), lasona (Bisaya), cebollas (Bicol)
• Cambodia: khtüm krâhââm
• Laos: hoom bwàx
• Thailand: hom, hom-daeng (central), hom-lek (central)
• Vietnam: hành ta, hành tăm, hành nén.

Origin and geographic distribution

South-western Asia is the primary gene centre of A. cepa. However, true wild plants do not occur. Based on the current natural distribution of its ancestral group (A. oschaninii O. Fedtschenko and its allies), it is assumed that the domestication of A. cepa started in Tadzhikistan, Afghanistan and Iran. The early history of the crop mainly relates to common onion forms. Shallot is derived from common onion by selection among naturally occurring variants, and the first reliable records of its existence date back to 12th Century France. Shallot spread around the world, presumably from Europe. It is now found from the equator to as far north and south as the polar circles. It is less important than common onion, except in the tropical lowlands at latitudes between 10 °N and 10 °S, where it predominates.

Uses

Shallot (bulb) is used as food, spice and seasoning. It stimulates the appetite. It is often used raw, sliced, mixed with soy sauce and eaten with roasted meat. Shallot can be mixed with other ingredients and after grinding used as a spice for meat or fish. It is used for pickling, cooking and frying. Young inflorescences are a popular vegetable where the climate does not preclude bolting (e.g. northern Thailand, highlands of Indonesia).

Shallot also has medicinal properties. Traditionally it is used to reduce fever and to cure wounds. In the latter case the bulb is sliced, mixed with coconut oil and salt, boiled and placed as a poultice on the wound. Shallot is also used to lower blood sugar levels and inhibit platelet aggregation by eating it raw or cooked, as an extract or powder.

Production and international trade

Shallot is economically an important crop in South-East Asia. Indonesian statistics indicate that 358 000 t shallots were produced from 65 000 ha in 1987; Thailand produced 239 300 t from 18 700 ha, and the Philippines produced 60 890 t from 7000 ha (including potato onion) in the same year. Shallot bulbs are traded fresh, fried or pickled.

Properties

Per 100 g edible portion shallot bulbs contain: water 88 g, protein 1.5 g, fat 0.3 g, carbohydrates 9 g, fibre 0.7 g, ash 0.6 g, Ca 36 mg, P 40 mg, Fe 0.8 mg, vitamin A 5 IU, vitamin B₁0.03 mg, vitamin C 2 mg. The energy value is 160 kJ/100 g. A high soluble solids content gives optimal quality for frying. In this respect "Sumenep" is the best Indonesian cultivar, with a soluble solids content of 25-27 °Brix. The range in other local and imported cultivars is from 15-20 °Brix. Flavour and pungency also vary among cultivars and depend on the content of S-alk(en)yl cysteine sulphoxides.

The 1000-seed weight is 3-3.5 g.

Description

• A biennial herb, usually grown as an annual from bulbs, up to 50 cm tall. Roots adventitious, 1-2 mm in diameter, 10-25 cm long, arising from the true stem. True stem very short, flattened, formed at the base of the plant in the form of a disk; pseudostem formed by the sheathing bases of successive leaves.
• Leaves 3-8, alternate, distichous, glaucous, produced in succession from the broadening stem apex, each arising as a ring which elongates to form the tubular leaf-sheath; leaf-blade cylindrical, light to dark green in colour, hollow, erect or slanting.
• Bulbs formed by the thickening of leaf-bases a short distance above the true stem; as a result of the rapid formation of lateral bulbs or shoots, clusters of 3-18 bulbs of the first and second order are formed; protective bulbcoat-leaves purplish, brownish or white; mature bulbs oblongoid, globular or oblate, up to 5 cm in diameter, very variable in shape, size, colour and weight.
• Scape 1-several, up to 65 cm long, erect, straight, terete, at first solid, later becoming hollow.
• Inflorescence a spherical umbel, 2-8 cm in diameter, protected by a membranous spathe which splits into 2-4 persistent papery bracts; umbel with 50-2000 individual hermaphrodite flowers; pedicel slender; flowers subcampanulate to urceolate; tepals 6 in 2 whorls, ovate to oblong, 3-5 mm long, greenish-white; stamens 6; ovary superior, 3-locular, style simple, shorter than stamens at anthesis.
• Fruit a globular capsule, 4-6 mm in diameter, splitting loculicidally, containing up to 6 seeds.
• Seed about 6 mm × 4 mm, black, wrinkled.

Growth and development

Shallot is commonly grown from a bulb containing 1-5 sprouts, each covered by scales forming separate concentric rings within the bulb. After bulb dormancy is over, the sprouts grow and emerge from the bulb, developing a cluster of 1-5 plants. New lateral shoots continue to develop resulting in a cluster of up to 18 plants after 7-8 weeks. Adventitious roots arise from the bases of the shoots. The leaves of each shoot grow successively from the very short true stem. The bases of the older leaves turn into sheaths covering the younger leaves, the blades of the older leaves ultimately die off and their sheaths form a pseudostem. At the time the top of the pseudostem lodges (7-10 weeks after planting), the plants have 4-5 leaf-blades only.

Bulbs are formed from the lower parts of the leaf-sheaths. This is the result of photosynthate mobilization from the leaf-blade to the base of the leaves. Bulbing starts from the outer leaf-sheaths, but as bulbing progresses, the leaf primordia form only fleshy scales (their blade development is aborted). When the bulb matures, the 3-4 outermost leaf-bases dry up, disappear or remain present as a papery skin. In the centre of the bulb, primordial sprout leaves are formed.

Under favourable environmental conditions, the apical meristem of the shoot ceases leaf production and bolts. The last internode of the true stem elongates, forming the flower stalk, which bulges in the lower part. A. cepa is a facultative cross-pollinator, the percentage of selfing amounting to 10-20%. Pollination is by insects. The cultivar groups Aggregatum and Common Onion are fully cross-compatible, bearing fertile hybrids.

Other botanical information

The great infraspecific variability within A. cepa is nowadays considered to be divisible into two large horticultural groups:

• cv. group Common Onion: bulbs large, normally single, plants reproduce from seeds or from seed-grown bulbils (sets).
• cv. group Aggregatum: bulbs smaller, several to many forming an aggregated cluster, plants reproduce vegetatively via lateral bulbs (daughter bulbs).

The variability within cv. group Aggregatum is still insufficiently understood. The potato or multiplier onion (United Kingdom), the ever-ready onion (United Kingdom), the Russian vegetatively propagated onion, the "Utrechtse Sint Jansui" (the Netherlands) and the "griselle" (France) are considered to fall into this group together with the shallot. Clear distinction of those forms is often difficult, and it seems better to refer to them by cultivar names.

The so-called potato or multiplier onion is also grown in South-East Asia (e.g. in Indonesia, the Philippines and Thailand). It forms fairly large oblate bulbs with numerous laterals enclosed by the outer scales. These laterals produce separate tops and bulbs in their second year of growth, and the number of bulbs formed from a single bulb varies from 3 to 20. They differ from shallots (but many intermediate forms exist) in their larger bulb size, their often somewhat flattened shape, and usually having fewer daughter bulbs (of the first order only) which remain enclosed by the skin of the mother bulb for a longer period than in shallot.

The recent development of propagation of shallots by seed has reduced the usefulness of the current distinction of the two cultivar groups within A. cepa.

Ecology

Tropical shallot requires an average day temperature of 20-26 °C and a daylength of at least 11 hours, whereas common onion prefers slightly lower temperatures and a daylength of at least 13 hours. Shallot can grow in almost all types of soil with a pH higher than 5.6, but it prefers well-drained alluvial clay soil. In Indonesia 70% of the crop is grown in the lowlands below 450 m. It needs a lot of water, but very wet conditions can cause bulbs to rot.

Propagation and planting

Shallot is commonly propagated by bulbs. Small bulbs of 4-5 g should be used, and to avoid problems of dormancy they should first be stored for 3-4 months. They are planted in beds of 1-1.2 m width and 0.6 m height with furrows of 0.4-0.5 m width between the beds. Planting distances vary from 18-20 cm between rows and 10-15 cm in the row. Planting must be shallow with the top of the bulb remaining visible.

Recent research indicates that propagation by means of seed is promising. It enhances the size and shape of the bulb and the health of the plant, and virus diseases are minimized. True seed is much cheaper than seed bulbs, but raising a crop from seed is more difficult. Yields are lower (mostly singles, i.e. only one bulb per plant) and quality is suboptimal (heterogeneous). If shallot is grown from true seed, seed is sown in a seed-bed at a rate of 25-50 g per 3-5 m², enough to plant 100 m²; after 5 weeks the seedlings are transplanted to the field.

Husbandry

Shallot is widely grown in paddy fields after the rice crop has been harvested. Common cropping patterns are rice-shallot-shallot(-shallot) or rice-shallot-capsicum pepper. Relay cropping with capsicum pepper is common practice in Central Java. One month after planting shallot, 4-6-week old capsicum seedlings are transplanted in between.

The recommended fertilizer rates for shallot in Indonesia, on alluvial clay after rice, are 10 t/ha of manure or 4 t/ha of compost, and 200 kg/ha of triple superphosphate as basal dressing during tillage. The second application at two weeks after planting consists of a mixture of 250 kg/ha of ammonium sulphate, 100 kg/ha of urea and 50 kg/ha of potassium chloride; the same mixture is side-dressed again two weeks later. If organic manure is hard to obtain, it may be substituted by the same mixture of inorganic fertilizer to be applied before planting together with the phosphate.

During dry weather, shallot has to be irrigated frequently (daily or even twice a day) by sprinkling 3-5 mm. Weeds are a serious problem and weeding by hand is done every 2 weeks.

Diseases and pests

During the rainy season, purple blotch (Alternaria porri) and anthracnose (Colletotrichum sp.) are the main problems everywhere. Other fungal diseases are basal rot (Fusarium oxysporum) transmitted with the planting material, leaf blight (Stemphylium sp.), and to a lesser degree Sclerotium rot and neck rot (Botrytis aclada). Viruses observed in shallot are the aphid-spread onion yellow dwarf (poty)virus (OYDV) and shallot yellow stripe (poty)virus (SYSV). It is not known to what degree the viruses depress yield. The virus problem may be overcome by visually inspecting the planting material in the field and destroying any affected plants, or by the development of true-seed shallot cultivars.

In Indonesia the most serious pest problem in the dry season is army worm (Spodoptera exigua). The caterpillars hide inside the hollow leaves. Control is by hand picking and frequent spraying with insecticides. Thrips (Thrips tabaci) is reported to be a serious pest of shallot in Thailand.

Harvesting

Harvesting takes place after the leaves have wilted, usually 60-70 days after planting in the lowlands, and 80-100 days in the highlands. The shallots are pulled out, tied into bunches of 1-2 kg and left to dry in the sun for 5-14 days (with the leaves on top to protect the bulbs).

Yield

The average fresh shallot yield in Indonesia is 5.9 t/ha. Under optimal conditions, the best farmers may obtain a maximum of 18 t/ha. The dry season crop yields better than the wet season crop. The latter is often harvested in a premature stage, about 55 days after planting, because of severe fungal damage on the leaves. However, the net return from the wet season crop is higher because of fewer applications of insecticides and higher market prices.

Handling after harvest

After drying, the shallots are tied into 2 kg bunches which are sold directly (80-90%) or stored as planting material (seed bulbs) for the next season (10-20%). The planting material is stored by hanging it on bamboo racks close to a fireplace. For long-distance transport, the dry leaves are cut off and the bulbs are transported in bags of 50-100 kg.

Genetic resources

Vegetatively propagated shallot germplasm is maintained at the Hebrew University of Jerusalem (Israel) and the Lembang Horticultural Research Institute (LEHRI), Bandung (Indonesia); materials are mainly short-day cultivars. Long-day cultivars are maintained by the Research Institute of Vegetable Growing and Breeding, Olomouc (Czech Republic).

Breeding

Local cultivars from Indonesia (Java, Bali, Lombok and North Sumatra), the Philippines and Thailand have been collected by the Lembang Horticultural Research Institute (LEHRI), Bandung (Indonesia). Evaluation of this germplasm collection led to the recommendation of some local cultivars, e.g. "Bima", "Maja Cipanas", "Medan", and "Keling". A local cultivar from Thailand ("Sri Saket") is also popular in Indonesia ("Bangkok") for dry-season planting. Planting material (seed bulbs) imported directly from Thailand (Srisaket Horticulture Experiment Station) gives the best results. Breeding objectives are resistance to leaf diseases and improvement of bulb quality and yield. Sources of resistance are sought in related species, cultivated (A. fistulosum L.) as well as wild (A. roylei Stearn). Population improvement and hybrid development for shallot grown from true seed is receiving much attention. Consumers prefer red, round and large bulbs; for this purpose, crosses have been made between shallot and onion, with promising results.

Prospects

Selection of virus-free planting material, improved disease control, better storage methods for consumption shallots, and commercial growing from true seeds seem to be feasible improvements in the near future.

Literature

• AVRDC, 1988a. Vegetable research in South-East Asia. Asian Vegetable Research and Development Center (AVRDC), Shanhua, Tainan, Taiwan. 242 pp.
• Currah, L. & Proctor, F.J., 1990. Onions in tropical regions. Bulletin No 15. National Resources Institute, Chatham Maritime, United Kingdom. 232 pp.
• Herklots, G.A.C., 1972. Vegetables in South-East Asia. George Allen & Unwin, London, United Kingdom. pp. 387-401.
• Jones, H.A. & Mann, L.K., 1963. Onions and their allies. Botany, cultivation and utilization. Leonard Hill, London, United Kingdom. 286 pp.
• Permadi, A.H., 1991. Penelitian pendahuluan variasi sifat-sifat bawang merah yang berasal dari biji [Preliminary study on the variation of characters in shallot grown from true seed]. Bulletin Penelitian Hortikultura [Horticultural Research Bulletin] 20(4): 120-131 (with English summary).
• Rabinowitch, H.D. & Brewster, J.L. (Editors), 1990. Onions and allied crops. Vols. 1 and 2. CRC Press, Boca Raton, Florida, United States. 273 pp. and 320 pp.

Authors

• Anggoro H. Permadi & Q.P. van der Meer