Onion

Description.

Crop details

The following are the crop details for onion.

Local Names: Kitunguu, (Swahili), Gitunguru (Kikuyu) Kitunguru  (Maragoli).

 

Scientific Name: Allium cepa

Order: Asparagales

Family: Amaryllidaceae

Subfamily: Allioideae

Genus: Allium

Species: A. cepa

Other names: bulb onion or common onion




General Information

Being a biennial vegetable onion is grown as an annual in temperate zones. The varieties that thrive in the tropics are effectively annuals because they can produce seed within the first year of growth. 

The plant's stem is a flattened disc with tubular leaves that form a pseudo stem where their sheaths overlap. There are 3-8 leaves per plant that are either erect or oblique. The plant produces p ink or white flowers cluster on its stalks. The bulbs are formed just above the flattened stem of the plant by overlapping leaves.

Onions may also be referred to as cultivars and these include red or purple onions, shallots, and spring onions or scallions. They are particularly suited to smallholder farming in most countries.

In the tropics onions can be grown year-round where irrigation is possible. 

The bulb is made up of layers, each of which corresponds to a leaf. They are generally oval, but their shape can vary, and they appear in clusters of 3-18 per plant. It is shielded by a membrane that transforms into a paper coat. Onion plants can grow to a height of 50 cm (20 in) and are grown as annuals, harvested after one growing season; however, in the tropics, onions can be grown year-round if irrigation is available.




Varieties

The history of ancestral onion species is not well documented. Ancient records of onion use span western and eastern Asia, so the geographic origin of the onion is uncertain. Yet, domestication likely took place in West or Central Asia. Onions have been variously described as having originated in Iran, western Pakistan and Central Asia.

 

Onion Varieties in Kenya

Red Creole: This is a popular standard variety in high demand because of its good keeping quality. It produces mainly single onions from transplants, red, flat-round, and with a pungent taste.

Red Tropicana: Red bulbing type

Red Tropicana F1 Hybrid: Produces large, red, thick flat onions with firm pungent flesh. It is highly productive and therefore demands high levels of management. It keeps well in a dry aerated store.

Bombay Red: It is a variety of dry and warmer conditions. It is small to medium-sized, globe-shaped, purplish red, and pungent.

Yellow Granex FI Hybrid: This is an early maturing high yielding attractive, thick flat onion with thin yellow scales. The flesh is medium firm, crisp, and mild in flavor. The shape and size are uniform leading to higher market prices, and the storage quality is good.

Texas early Grano: This is a fresh market, early maturing variety (100 - 120 days) with a rather short shelf life. It is yellowish, mild, and not very pungent. The bulbs are high-top shaped with dry yellow scales. It is a heavy yielder for high-altitude regions. Ideal for fish salads.

White Creole: This is a white variety normally used for dehydration.

Green bunching: non-bulbing spring onion. It has attractive tasty dark green leaves. It is an early and highly productive onion grown for stems rather than bulbs. It is tolerant to sun scotch and it is recommended for salads and fresh markets.

 

Onion Varieties in East Africa

In Tanzania, Red Bombay is the most popular onion variety. Its production is highest in March, May, and December and is predominantly sourced from Mang’ola.

Other popular varieties grown in the East Africa region include Red Pinoy F1, Red Creole, Bombay Red, and Texas Early Grano.




Climate Conditions, Soil, and Water Management

In temperate zones, onion is a cool-season biennial and is tolerant to frost. They produce bulbs with growing day lengths. Optimum temperatures for plant development are between 13 and 24°C, although the range for seedling growth is narrow, between 20 and 25°C. High temperatures favor bulbing and curing. In the tropics, only short-day or day-neutral onion varieties will form bulbs. These thrive in warm to hot climates of 15-30°C. If the temperature greatly exceeds that required for bulbing, maturity is hastened and bulbs do not grow to maximum size, consequently lowering the yields. 

Onions can be grown on any fertile, well-drained, non-crusting soil. The optimum pH range is 6.0 to 6.8, although alkaline soils are also suitable. Onions do not grow well in soils below pH 6.0. On light sandy soils, irrigation is necessary. Irrigation could be either overhead or on a drip. Onions at the bulbing stage need a substantial amount of water, but excessive moisture must be avoided during the growing season. Avoid the application of fresh manure to the crop, as this will cause the plants to develop thick necks and too many leaves at the expense of bulb formation.




Uses

The bulb is an edible vegetable and is the most commonly used part of the onion, usually consumed after cooking although it can be eaten fresh. The stems and leaves are also edible. In general, onions are used for salads (bunching onions or sliced full-grown bulbs), pickling (e.g. silver skin onions), cooking (such as in soups), and frying (for example, with meat).  It also plays an important role in traditional medicine (e.g. as a diuretic).




Planting Procedure

The planting procedure for onion is as follows:

A grower must consider several factors when planting onions, among them are onion varieties. Onions can be planted in three different ways: by sets transplants or direct seeding. Sets are grown from seeds, producing small bulbs before going dormant. 

Planting sets and transplants can be advantageous because they are ready to harvest one to two months earlier than onions planted directly. Planting in sets and transplanting is much more expensive than direct seeding. 

Harvests can be completed earlier in the year when using transplants or sets, which may result in higher profits if the market demand for the produce is higher. Planting sets or transplants necessitate the use of specialized planting equipment. It is necessary to consider the cost of acquiring the necessary equipment.

In some environments, using sets and transplants exposes the onion to environmental pressures for a shorter period of time, reducing the need for weed and pest control. A precision seed drill and other onion handling equipment are required for direct seeding. For direct seeding, the soil must be tilled, smoothed, and leveled prior to seeding. 

The ideal soil site is well-drained and has a nearly neutral pH (6.6-6.8). (Growing Onions). Direct seeding requires that each seed be planted at the same depth below the soil surface in order for it to germinate and grow uniformly. Typically, seeds are planted 1 inch deep.




Planting Systems

Nursery seeding and transplanting is the most common and practical option in the tropics. Transplants normally have 3 to 5 well-formed leaves at transplant time. Roots are pruned during planting, in order not to be bent upwards when transferred to the field. This facilitates early establishment of the plant.

Any germinated bulb of the above-mentioned varieties would produce 3-6 good size bulbs in about 3 months when planted with the rains. Choose only healthy bulbs for propagation.

Sets are used in some areas in the temperate zones to ensure large bulb size and uniform maturity. Sets are small dry bulbs, approximately 12 mm in diameter, which have been produced the previous season by seeding thickly or growing under conditions that favor rapid bulbing.

Direct seedling is possible and gives excellent results where herbicides can be used and the season is sufficiently long to provide early pre-bulbing growth. In the tropics this method is impractical due to enormous weeding costs in an organic system.




Husbandry

Do not plant onions after the field has been planted with other Allium plants (e.g. garlic). Mulching onions with composted leaves and straw is highly recommended to maintain soil organic content, prevent soil-borne diseases, and suppress weeds. Planting onions in raised beds improve drainage and prevents damping-off diseases.

Weeding and harvesting are mostly done by hand, although chemical weed control is possible but not organic. Weed control is a critical part of onion production. Common weeds that affect onion and garlic include yellow nutsedge, field morning glory, cheeseweed, chickweed, henbit, lambs quarters, marestail, pigweed, purslane, shepherdspurse, annual bluegrass, barnyardgrass, and foxtail. Onion transplants and seedlings do not compete well with weeds, so hand cultivation or herbicides are necessary if weeds are a problem. Crop rotation is important to avoid the build-up of pests and diseases such as nematodes, Sclerotium and Fusarium.




Harvesting

Harvesting takes place 90-150 days after sowing. Onions are ready for harvest when the leaves collapse. Alternatively, the leaves can be bent over and left to dry for 10-12 days. The crop is pulled out by hand and kept for some days in the field with the bulbs covered by the leaves (= windrowing). The leaves are then cut off and the mature bulbs are bagged or packed in crates if they are to be stored.

Freshly harvested onions are dormant and will not sprout for a variable period (this depends on the variety). Storage will extend the dormant period. Sprouting will increase in storage temperatures above 4.4°C. It will decrease again as temperatures exceed 25°C.

 

Postharvest Treatment

Onion bulb curing

Following harvest, bulbs are cured to heal wounds and for drying. Curing improves postharvest handling characteristics and limits the entry of organisms through the pseudostems or injured tissues. It also helps the formation of attractive, well-colored, intact outer skins. When weather permits, bulbs are cured in the field for several days, depending on environmental conditions, until the neck has sealed, the outer scales are papery and the wounds have healed.

For field or indoor curing at ambient temperature, bulbs are windrowed to air-dry or placed into well-ventilated boxes or bags, respectively. Onions can also be cured with forced circulation of warm (30°C), low-humidity air through bins or piles of onions placed on slatted floors for 12-24 h. During curing, onion can lose as much as 5% of its initial harvest weight.

 

Onion storage

Green onions should be stored at 0°C and 95% RH with a storage life of up to 3 weeks. At 5°C, storage life may be limited to 1 week. Best storage occurs after bulbs have been cured to seal bulbs and heal damage as described above. Onion bulbs do not heal cuts and surface wounds as successfully as other crops such as Irish potato, so mechanical damage during harvest and handling should be minimized before storage.

Decay, sprouting, and rooting cause losses during storage. These losses can be minimized by storage at or near 0°C and 65-70% RH. Storage at high temperatures (25-35°C) may also be satisfactory.

Dry matter and moisture are lost during storage. Most bulbs shrink in size due to respiratory losses and carbohydrate translocation from the outer to inner scales or sprouting roots and shoots. This reallocation of resources causes the succulent outermost scales to gradually desiccate, becoming a dry protective layer, further reducing water loss from the inner scales. 

Bulb diameter decreases because of the net loss of succulent scales over time. Onion respiration rates are temperature dependent, decreasing with refrigerated storage. Respiratory heat must be removed by ventilation or refrigeration. Relative humidity influences storage life by reducing shrinkage from water loss. Modified atmosphere storage with elevated CO2 and reduced O2 can extend onion storage life.

As a general rule, long-day types with high solids have longer storage potential than short-day types with low solids. Pungent cultivars generally have longer storage potential than sweet. A fully cured pungent bulb may be stored under optimal conditions for up to 1 year. In contrast, a sweet cultivar may only last 2-3 months under similar conditions.

 

Bulb rest and dormancy

Onion bulb dormancy is complex with different phases. Onion bulbs first enter a state of rest at harvest maturity that lasts 6-8 weeks. After the rest period, a natural dormancy period begins and bulbs will not sprout or grow because inhibitors synthesized in green leaves earlier in development remain. The inhibitors are gradually destroyed with time. Therefore, it is important to have normal senescence of onion leaves to ensure translocation of inhibitors to the bulb to improve storage life and reduce early sprouting. The change from rest to dormancy is gradual and highly dependent on the cultivar. Dormant bulbs will not sprout at optimal storage temperatures. Once dormancy has passed, root emergence occurs, followed by leaf shoots at favorable temperatures and moisture conditions.

 

Bulb sprouting and sprout inhibition

Sprouting is optimum at 10-15°C. Sprouting is inhibited at both low and high temperatures as explained above. For long-term storage, a spray inhibitor such as maleic hydrazide (MH) is sometimes used, but it is banned in many countries. MH at a concentration of 2500 ppm is applied at 500 L water/ha. Spray inhibitors are generally applied in the field 1-2 weeks before harvest. If applied too soon, foliage injury occurs and when applied too late, foliar absorption is insufficient to be effective. To improve absorption, applications are made when most of the foliage is still green and when dew is not present. Onions treated with MH and held at a temperature between −1°C and 0°C and 65-70% RH can be stored for as long as 6-7 months without sprouting. Bulb rooting and sprouting are inhibited by gamma irradiation and controlled-atmosphere storage at reduced O2, and continuous removal of newly formed roots tends to delay sprout emergence.




References

Boyhan, G. E. & Kelley, W. T. (Eds.) (2008). Onion production guide. University of Georgia Cooperative Extension. Available at: http://www.caes.uga.edu/extension/thomas/anr/documents/onionproductionguide_b1198_2.pdf. [Accessed 01 March 15]. Free to access. CABI Crop Protection Compendium. (2013). Allium cepa (onion) datasheet. Available at: http://www.cabi.org/cpc/datasheet/4239. [Accessed 01 March 15]. Paid subscription required. Drost, D. (2004). Onions in the garden. Utah State University Cooperative Extension. Available at: http://extension.usu.edu/files/factsheets/onionsf.pdf. [Accessed 01 March 15]. Free to access. Schwartz, H. F. & Mohan, S. K. (Eds.) (2008) Compendium of Onion and Garlic Diseases and Pests.Second Edition. American Phytopathological Society Press. Available at: http://www.apsnet.org/apsstore/shopapspress/Pages/43573.aspx. Available for purchase from APS Press.


Common Pests and Diseases

Diseases

Category : Fungal

Black mold Aspergillus niger

Symptoms
Post-harvest black discoloration at neck; lesions on outer scales; black streaks under outer dry scales; entire surface of bulb turning black and shriveling
Cause
Fungus
Comments
Wash hands thoroughly after coming into contact with fungus
Management
Treat seeds with appropriate fungicide prior to planting to reduce rot in mature bulbs; avoid bruising bulbs during and after harvest; storing at temeperatures below 15°C (59°F) prevents mold from spreading but it will resume once temperature increases

Botrytis leaf blight Botrytis squamosa

Symptoms
Small white lesions with light green halos which may expand slightly as they age; in prolonged periods of moisture fungus may develop rapidly and cause leaf blighting
Cause
Fungus
Comments
Disease emergence favors high humidity and warm temperatures; fungus survives on piles of crop debris or in soil; older leaves more susceptible to blighting than younger leaves
Management
Plant onions in single rows allowing at least 30 cm between plants to promote good air circulation and quick drying of foliage after rain; time irrigation to allow plants time to dry out sufficiently; apply appropriate fungicide sprays when plants have at least five true leaves and early symptoms of disease

Downy mildew Peronospora destructor

Symptoms
Pale spots or elongated patches on leaves; gray-purple fuzzy growth on leaf surface; leaves turning pale then yellow; leaf tips collapsing
Cause
Fungus
Comments
Disease emergence favored by cool temperatures and leaf wetness
Management
Avoid planting infected sets; rotate crops to non-allium species for 3-4 years; plant in well-draining areas and do not overcrowd plants; destroy all infected crop debris; apply appropriate foliar fungicides taking care to apply thoroughly to waxy leaves

Fusarium basal plate rot Fusarium oxysporum

Symptoms
Curving, yellow or necrotic leaves; necrosis begins at leaf tips and moves downward; wilting plants; infected bulbs may be brown and watery with rot spreading from stem plate to basal leaves; stem plates may have brown discoloration
Cause
Fungus
Comments
Disease emergence favors moderate to high temperatures
Management
Rotate with non-susceptible crops for at least 4 years, plant resistant onion varieties

Fusarium damping-off Fusarium oxysporum

Symptoms
Rotting seeds that are covered in mold; discolored root tips which may be pink, tan, yellow, red or black; slowly growing seedlings which wilt and die
Cause
Fungus
Comments
Fungus survives in soil and disease emergence is favored by moist to wet soil
Management
Plant only disease-free seed; treat seed with fungicide; rotate crops with cereals or grasses to reduce levels of pathogen in soil; steam treatment or fumigation of soil can help reduce levels of Fusarium in the soil

Pink root Phoma terrestris

Symptoms
Light pink roots which darken and turn purple; roots become transparent and water soaked; plant may look like it has a nutrient deficiency; infected seedling may die; stunted plants with undersized, shriveled bulbs
Cause
Fungus
Comments
Fungus colonizes plant through root tips; fungus can survive in soil down to a depth of 45 cm (17.7 in)
Management
Disease is most severe when onions are planted continuously or in a 1-year rotation, a rotation of 3-6 years is preferred; plant more resistant varieties; solarization and/or fumigation can help reduce the levels of pathogen in the soil

Purple blotch Alternaria porri

Symptoms
Small water-soaked lesions lesions on leaves or stalk with white centers; which enlarge to become zonate and brown to purple in color with red or purple margin surrounded by yellow zone; large lesions may coalesce and girdle leaf, killing any tissue between the lesions and the leaf tip; severely infected foliage may die
Cause
Fungus
Comments
Disease emergence favored by wet foliage, with sporulation occuring during the night during periods of high humidity
Management
Cultural controls include long rotations with non-hosts and the reduction of leaf wetness by planting in well-draining soil and timing irrigation to allow plants to dry adequately during the day; some fungicides are effective at controlling the disease but should be rotated for optimal control

Rust Puccinia porri

Symptoms
Small white flecks on leaves and stems which develop into circular or elongated orange pustules; severe infestations can cause leaves to yellow and die
Cause
Fungus
Comments
Favors high humidity but low rainfall; spores can be transported over long distances by wind
Management
No resistance known; use only disease-free seed and plant in well-draining soil; control weeds around crop; apply appropriate protective fungicide

Smut Urocystis colchici

Symptoms
Dark, thickened lareas on cotyledons (seed leaves) which may become large and cause leaves to bend downwards; raised blisters may be present on the base of scales in older plants; lesion mature and become covered in black powdery fungal masses; plant growth stunted; death of plant occurs within 3-4 weeks
Cause
Fungus
Comments
Smut can persist in soil for many years and is mainly introduced through infected sets and transplants; spores can be spread by wind, on equipment or in irrigation water
Management
No resistance to disease known in onion; plant onions during periods which promote rapid growth; plant only health sets and transplants - if smut is present in the soil they will not become infected

White rot Sclerotium cepivorum

Symptoms
Older leaves yellowing; stunted growth; death of all leaves; fluffy white growth on base of bulb which spreads up bulb to storage leaves
Cause
Fungus
Comments
Fungus can survive in soil for 20 years and is one of the most damaging diseases of Allium crops worldwide, causing major crop losses
Management
Fungicide treatment may not be effective at controlling white rot under conditions which are favorable to the fungi's development and control may have to rely on cultural methods: avoid transferring soil or plant material between sites; treat seeds with hot water prior to planting; use a long term rotation with non-allium crops; apply appropriate fungicides if available

Category : Bacterial

Leaf streak and bulb rot Pseudomonas viridiflava

Symptoms
Water-soaked, dark green oval lesions or streaks on leaves; tipburn of leaves; dark spots on wrapper scales of bulbs; reddish-brown discoloration of inner scales; rot developing in ring-like pattern
Cause
Bacterium
Comments
Little is known about the pathogen; greatest damage occurs during winter; rapid spread of disease on infected plants is promoted by rainfall
Management
Avoid fertilizing plants during winter' apply appropriate bacteriacidial sprays

Category : Viral

Onion yellow dwarf Onion yellow dwarf virus (OYDV)

Symptoms
Yellow streaks on bases of of first leaves; all leaves which emerge after infection have yellow streak or are completely yellow; leaves may be flattened or crinkled; bulbs are undersized; flower stalks yellow and twisted; flower clusters small and seed is of poor quality
Cause
Virus
Comments
Transmitted by several species of aphid, including the peach aphid; virus is not spread via seed or pollen
Management
Controlling aphids by applying insecticides is not effective due to the short amount of time aphid needs to transmit virus; other control methods include: planting sets or transplants which were produced in areas free of virus; growing plants from seed; removing any infected plants and planting more tolerant varieties

Category : Oomycete

Pythium seed rot (Damping-off) Pythium irregulare

Symptoms
Seeds water-soaked, mushy and decomposing; infected roots are gray and water-soaked; seedlings that have already emerged prior to infection collapse and die; older plants that become infected become severely stunted
Cause
Fungus
Comments
Disease emergence favors high soil moisture and cool temperatures
Management
Control of disease is dependent on minimizing soil moisture: break up compacted soil; plant in well-draining areas or raised beds; treat seeds with appropriate fungicides prior to planting

Pests

Category : Mites

Bulb mites Rhizoglyphus spp.
Tyrophagus spp.

Symptoms
Stunted plant growth; reduced stand; bulbs rotting in ground or in storage; pest is a cream-white, bulbous mite <1 mm in length, which resembles a pearl with legs
Cause
Arachnid
Comments
Damage to plants by bulb mites allows secondary invasion by other pathogens and can cause bulb rots
Management
Do not plant successive crops of onion or garlic in same location; allow field to fallow to ensure that any residual organic matter decomposes completely - crop residues can harbor mite populations; treating garlic seed cloves with hot water prior to planting may help reduce mite populations

Category : Insects

Leafminers Lyriomyza spp.

Symptoms
Thin, white, winding trails on leaves; heavy mining can result in white blotches on leaves and leaves dropping from the plant prematurely; early infestation can cause yield to be reduced; adult leafminer is a small black and yellow fly which lays its eggs in the leaf; larave hatch and feed on leaf interior
Cause
Insects
Comments
Mature larvae drop from leaves into soil to pupate; entire lifecycle can take as little as 2 weeks in warm weather; insect may go through 7 to 10 generations per year
Management
Check transplants for signs of leafminer damage prior to planting; remove plants from soil immediately after harvest; only use insecticides when leafminer damage has been identified as unnecessary spraying will also reduce populations of their natural enemies

Onion maggot Delia antiqua

Symptoms
Stunted or wilting seedlings; plant will commonly break at soil line if an attempt is made to pull it up; if infestation occurs when plants are bulbing, bulbs will be deformed and susceptible to storage rots after harvest; adult insect is a greyish fly which lays white, elongate eggs around the base of the plant; the larvae that emerge from the eggs are tiny and white and bore into the onion plant; mature larvae are about 1 cm (0.4 in) long with feeding hooks
Cause
Insect
Comments
Females can lay several hundred eggs during their 2-4 week lifespan; insect overwinters as pupae in the soil
Management
Management of onion maggots is heavily reliant on good sanitation; all onion bulbs should be removed at the end of the season as maggots will die without a food source; commercial onion growers must often rely on the application of appropriate granular insecticides and, in some cases, insecticide sprays are also required; home gardeners should try to remove any volunteer wild onion and chive plants as these can act as an infection source; floating row covers may provide protection by preventing females from laying eggs around the plants

Thrips (Onion thrips, Western flower thrips) Thrips tabaci
Frankliniella occidentalis

Symptoms
Discolored, distorted tissue; scarring of leaves; severly infected plants may have a silvery appearance
Cause
Insect
Comments
Thrips are most damaging when they feed on onions at the early bulbing stage of development; both onion thrips and western flower thrips have an extensive host range and can be introduced to onion from other plants
Management
Natural enemies include some species of predatory mite, pirate bugs and lacewings; avoid planting onion in close proximity to grain fields as thrips populations build up on these plant in the spring; overhead irrigation of plants may help reduce thrips numbers; apply appropriate insecticides at first sign of thrips damage
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