Onion – Allium Cepa

Onion (Allium cepa) is an important vegetable crop in the Kenya, being one of the most widely consumed vegetable. It is grown on both large and small scale farms, in greenhouses and in open gardens.

Onions are served as food when cooked, can be eaten raw or be used to make pickles.

Varieties

In Kenya, there are various onion varieties. These include both the hybrid and the local varieties.

Ecological Requirements

Onions can be grown in a wide range of ecological conditions, both in greenhouses and in open fields.

• Soil– the crop thrives in a well-drained fertile soil, with an optimum pH range of 6.0 – 7.5. Onions are very sensitive to highly alkaline soils. The soil should also be weed free.
• Rainfall– onions can grow well in places which receive an annual rainfall of between 650-800mm. Irrigation should however be done to ensure consistency in moisture content.
• Temperature– an optimum temperature of 13-24 and 18-26 degrees centigrade is essential for vegetative and bulb formation stages of the crop growth and development. Seeding stage requires an optimum temperature of 20-25 degrees centigrade.
• Altitude– the crop can do well up to an altitude of 2500M ASL.

Planting

Propagation is majorly by seeds, which are first raised in a seedbed.

Raising seedlings

Procedure

• Choose a place where no onion family crop has been cultivated for at least 2 years.
• Construct raised beds, 1 meter wide.
• Drench soil with a mixture of LOYALTY 700WDG 10g + PYRAMID 700WP 100g+ OPTIMIZER 20mlin 20 litres of water. This helps to eradicate soil pests and diseases, break seed dormancy and promote uniform germination.
• Form shallow furrows.
• Sow seeds and cover lightly with soil.
• Mulch and water the bed.

Seeds germinate within 7-10days after sowing.

After germination, mulch is removed.

Transplanting

The seedlings are ready for transplanting 6-8 weeks after sowing.at this time, majority of the seedlings necks are pencil-size , about 65-80mm in diameter, 14-30cm tall and have 4 to 5 leaves.

Transplanting should be done early in the morning from 6-10am or late evening 4-6pm.

Prior to transplanting, the nursery bed should be watered to saturation in order to avoid damaging the seedlings

To overcome transplanting shock, seedlings should be sprayed with OPTIMIZER 10ml/20l

Seedlings are pulled from the wet nursery bed and transplanted in the main field soonest possible at a spacing of 8*10cm.

Pest & Disease Management

Pests

• Cutworms– these are caterpillars which live in the soil. They cut and eat the stems of young seedlings and trans-plants. Heavy infestations can cause total crop loss, if not controlled.

Drench soil with PROFILE 440EC 60ml/20l or PENTAGON 50EC 20ml/20l

• Thrips- these are the most important insect pests of onion crops, which cause significant economic losses. They cut the epidermis of the leaves or stem and suck the plant sap creating white silvery patches on the infested/deformed leaves.

Spray ALONZE 50EC 5ml/20l or PROFILE 440EC 30ml/20l or DEFENDER 25EC 40ml/20l

• Onion flies– the larvae, which are creamish white maggots feed on the lateral roots, then tunnel into the taproot and sometimes bore into the base of the stem. Infested leaves turn bluish and eventually wilt. The plant becomes shriveled and dies. The maggots are also found inside developing onion bulbs. Their feeding exposes the plant to infection by diseases like soft rot.

Spray KINGCODE ELITE 50EC 10ml/20l or LEXUS 247SC 8ml/20l or PRESENTO 200SP 5g/20l

Diseases

• Damping off– this is a soil borne disease which attacks the seeds and seedlings. Infected seedlings may fall over and die due to breakdown of plant tissues at the soil level. Sometimes this disease may occur even before the seedlings emerge.

Spray CHANCETYL ELITE 800WDG 50g/20l or MILLIONAIRE 690WDG 40g/20l or drench PYRAMID 700WP100g/20l

• Downy mildew– Diseased leaves turn pale-green, then yellow, and collapse and eventually die. The pale-green and yellow stage is characterized by oval-shaped lesions that often become infected with other diseases. This disease is favored by cool and humid weather.

Spray GEARLOCK TURBO 250WP 25g/20l or FORTRESS GOLD 720WP 40g/20l or TRINITY GOLD 425WP 50g/20l

• Purple blotch– infection causes the development of small water soaked lesions with whitish centres, mostly on the older leaves. These lesions enlarge as infection progresses and become purplish with light yellow concentric rings on the margins. Leaves may turn yellowish-brown, lose erectness and eventually wilt, if the disease is not controlled.

Spray COLONIZER 440WP 50g/20l or ABSOLUTE 375SC 10ml/20l or TRINITY GOLD 425WP 50g/20l

• Onion rust- on infection, whitish to yellow spots appear on the leaves, which enlarge as disease progresses. Over time, they become orange and develop into black lesions. Stunted bulb formation occurs in heavy infections. This causes significant economic losses.

Spray MILLIONAIRE 690WDG 40g/20l or COLONIZER 440WP 50g/20l or MILESTONE 250SC 10ml/20l

• Botrytis neck rot– this disease affects stored onions, although it comes from the soil. On infection, the neck of the onion becomes watery, brown then hardens to wizened papery leaves. A grey mold may grow between the onion layers in cases of high humidity.

Spray EXEMPO CURVE 250SC 15ml/20l or EXPLORER 3SL 10ml/20l or IPRODE 500SC 10ml/20l

• Pink rot- this is a soil borne disease which is easily recognized, because, on infection, the roots turn pink or maroon. In severe cases of infections, roots may die and the plants become weakened or stunted. The main effect of pink rot disease is reduced bulb size.

Spray MILLIONAIRE 690WDG 40g/20l or EXEMPO CURVE 250SC 15ml/20l or RIMETA GOLD 300SC 40ml/20l

• Onion smut– this is a soil-borne disease which infects the cotyledon as it grows through the soil. In most cases, seedlings survive this initial infection and characteristic black streaks and blisters appear on the leaves and small bulbs later in the growing season as the fungus moves from the infected cotyledons to the younger leaves.

Spray COLONIZER 440WP 50g/20l or MILESTONE 250SC 10ml/20l or RANSOM 600WP 15g/20l

• Fusarium basal rot– initial symptoms on infection are yellowing of leaves and dieback of tips. As the disease progresses, the whole plant may collapses and, if pulled, it comes out without any roots attached because they have decayed. The basal plate of the onion is usually pinkish-brown. If infection occurs late in the season, the symptoms may not show up until the onions are in storage.

Drench soil with PYRAMID 700WP 100g/20l

• Botrytis leaf blight– initially, symptoms on the leaves appear as greyish-white, oval-shaped spots which are usually surrounded by a silvery white halo with uneven margins. The centres of most of the spots become sunken and straw- colored. In severe cases, tips of the leaves begin to dieback.

Spray EXEMPO CURVE 250SC 15ml/20l or IPRODE 500SC 10ml/20l or EXPLORER 3SL 10ml/20l

• White rot– initially, infection causes yellowing and dieback of the leaf tips. The infected leaves collapse. As infection progresses, bulbs and roots develop white, fluffy molds and soft rots. Masses of tiny black fungal growth can also be seen within these molds. Infected bulbs can rot in storage boxes and spread infection to healthy ones.

Spray MILLIONAIRE 690WDG 40g/20l or RIMETA GOLD 300SC 40ml/20l or EXEMPO CURVE 250SC 15ml/20l

Nutrition & Nutritional Deficiency

For optimal production, onion crop should be supplied with adequate nutrients. This can be achieved through both basal and foliar fertilizer application.

Basal fertilizer are absorbed by the crop through the roots. They include the following

• DAP- this is applied during planting and essentially supplies Phosphorous among other elements to the younger growing crop.
• CAN, NPK & UREA-these are used for top dressing, after the crop has developed. They supply Nitrogen among other nutrient elements.

Since onions require a higher amount of Nitrogen, top dressing should be done at least twice.

Foliar fertilizer are absorbed by the crop through its foliage tissues. They include the following;

• OPTIMIZER-this is important in building the crop’s immunity, managing stress, supplying both micro & macro elements, among other benefits. It can be applied at any growth stage of the crop.
• GOLDCHANCE SERIES-these are used to nurture the crop in its different stages of growth and development.
• DIMIPHITE- this is essential in bulb formation.

Deficiencies

• Nitrogen-deficiency causes leaves to turn yellowish green, upright curled, wilted and dwarf. The tissue above bulbs become soft at maturity.

Correction; spray LAVENDER 20ml/20l 0r GOLDCHANCE SUPER GROWTH 50g/20l

• Phosphorous– deficiency leads to slowed plant growth. Leaves becomes light green and bulbs have few dried outer peals. Tips of older leaves appear scorched.

Correction; spray LAVENDER 20ml/20l or GOLDCHANCE SUPER START 50g/20l or DIMIPHITE 10ml/20l

• Potassium– deficiency symptoms mostly develop on the young leaves because Potassium is very mobile. Leaves become dark green and erect while the tips burn. Bolting may occur. The older leaves turn yellow and necrotic.

Correction; spray DIMIPHITE 10ml/20l or GOLDCHANCE FLOWERS & FRUITS 50g/20l

• Iron- deficiency leads to complete yellowing of young leaves. This starts out as an interveinal chlorosis then turns into an overall chlorosis, eventually bleaching the entire leaf.

Correction; spray EURO GOLD PLUS 50g/20l

• Zinc– deficiency restricts plant growth, while leaves show interveinal necrosis. In the early stages of deficiency the younger leaves become yellow and pitting develops in the interveinal upper surfaces of the mature leaves. If deficiency is not corrected, these symptoms develop into an intense interveinal necrosis but the main veins remain green.

Correction; spray ZINC GOLD 10ml/20l

Physiological disorders

• Bolting– this is the production of flower stalks at the expense of bulb formation and enlargement.
• Onion greening– this is caused by exposure of bulbs to sunlight whereby they form chlorophyll.
• Sunscald- this occurs when bulbs are exposed to direct sunlight during high temperatures.
• Pinking-this usually occurs when bulb is bruised, especially during harvesting.

Weed Control

For successful onion production, managing weeds is very critical. This is because weeds compete with the crop for nutrients, water, space, among other factors. They also harbour pests and diseases.

The following herbicides make weed control in onion field easier and effective;

• CLAMPDOWN 480SL 200ml/20l- this is used during land preparation. It kills all types of weeds.
• PREDATOR 340EC 20ml/20l- this is a selective herbicide which is applied to soil immediately after sowing seeds, before seedlings emerge and /or before transplanting the seedlings.
• COMMANDER 240EC 50ml/20l- this is a selective herbicide which should be applied 2-3 weeks after transplanting the seedlings.

NOTE; whenever conducting any foliar spray, it is recommended to always mix the product (insecticide, fungicide, foliar feed or herbicide) with INTEGRA 3ml/20l. This is a sticker, spreader and penetrant which improves the efficacy of the product.

Maturity, Harvesting & Post-Harvest Handling

It takes approximately 4-5 months for onions to mature, depending on the variety as well as well the environmental conditions.

The optimum time to harvest onions is a week after 50% of the leaves have fallen. Signs of maturity include yellowing of leaves, bulbs turning red and attaining an optimum size, among others. The bulbs however continue growing even after the leaves have fallen.

Harvesting is done by pulling the bulbs or using mechanical harvesters in case of large scale farming.

Curing Onion

The curing process involves exposing the onions to mild temperatures in a dry and ventilated area. They can also be dried in warm shades for some time. The process is very important for long-term storage of onions.

As the onions cure, the skins dry into papery wrappers, pungent compounds replace the sugars and the necks at the top of the bulbs come together, sealing out moisture and microorganisms.

While in storage, onions are prone to sprouting if the storage conditions are not good. They should thus be dried thoroughly.

Storage

Onions are stored in net bags, crates, in pallet boxes, or in bulk bins. When stored in bins or boxes, there should be at least some vent space. Bags of onions are frequently stored on pallets and should be stacked to allow proper air circulation.