Onion – Allium Cepa

In Kenya, the onion (Allium cepa) is a significant vegetable crop and is highly popular among consumers. It is cultivated in various settings, including large and small-scale farms, greenhouses, and open gardens.

Onions are versatile and can be enjoyed in multiple ways. They can be cooked and served as a delicious food, consumed raw, or utilized in the preparation of pickles.

Varieties

Kenya boasts a diverse range of onion varieties, encompassing both hybrid and local types.

Hybrid varieties Local varieties
Red Coach F1 

Malbec F1

Red Pinnoy F1

Jambar F1

Neptune F1

Red Passion F1

Rasta F1

Red Connet F1

Red Creole 

Red Bombay

Texas Grano

Ecological Requirements

Onions have the ability to thrive in a wide range of ecological conditions, whether it be in greenhouses or open fields.

Soil: Onions prefer well-drained and fertile soil with an ideal pH range of 6.0 to 7.5. They are particularly sensitive to highly alkaline soils, so it is important to avoid such conditions. Additionally, it is beneficial for the soil to be free from weeds.

Rainfall: Onions can flourish in areas with an annual rainfall between 650-800mm. However, to maintain consistent moisture levels, irrigation should be implemented.

Temperature: The optimal temperature for onion growth and development varies across different stages. For vegetative and bulb formation stages, temperatures between 13-24 and 18-26 degrees Celsius are desirable. During the seeding stage, an optimum temperature of 20-25 degrees Celsius is recommended.

Altitude: Onions can thrive at altitudes up to 2500 meters above sea level (ASL).

Planting

The primary method of propagating onions is through seeds, which are initially sown and nurtured in a seedbed.

Raising seedlings

Here is a rephrased version of the procedure:

  1. Select a location where no crops from the onion family have been grown for a minimum of 2 years.
  2. Create raised beds that are 1 meter wide.
  3. Apply a soil treatment mixture consisting of 10g of LOYALTY 700WDG, 100g of PYRAMID 700WP, and 20ml of OPTIMIZER in 20 liters of water. This solution helps eliminate soil pests and diseases, breaks seed dormancy, and promotes uniform seed germination.
  4. Create shallow furrows in the prepared soil.
  5. Sow the onion seeds and lightly cover them with soil.
  6. Apply mulch to the bed and provide adequate watering.
  7. Seeds will typically germinate within 7-10 days after sowing.
  8. Once germination occurs, remove the mulch from the bed.

Transplanting

The seedlings reach a suitable stage for transplanting approximately 6-8 weeks after sowing. At this point, the majority of seedlings will have necks resembling the size of a pencil, measuring around 65-80mm in diameter. They will be 14-30cm tall and possess 4 to 5 leaves.

For the transplantation process, it is recommended to carry it out either early in the morning, between 6-10 am, or in the late evening, from 4-6 pm.

Before commencing with the transplanting, it is essential to thoroughly water the nursery bed until it reaches a state of saturation. This step helps prevent damage to the delicate seedlings.

To minimize transplanting shock, it is advisable to spray the seedlings with a solution of 10ml of OPTIMIZER per 20 liters of water.

The seedlings are carefully pulled from the moist nursery bed and promptly transplanted into the main field. It is recommended to space the seedlings at an interval of 8 by 10 centimeters.

Pest & Disease Management

Pests:

  1. Cutworms: These are caterpillars that reside in the soil and pose a threat to young seedlings and transplants by cutting and consuming their stems. If left uncontrolled, severe infestations can result in complete crop loss.

Solution: Apply a soil drench of either 60ml of PROFILE 440EC or 20ml of PENTAGON 50EC per 20 liters of water.

  1. Thrips: These insects are a major pest for onion crops and can cause significant economic damage. They cut the epidermis of leaves or stems and extract plant sap, resulting in white silvery patches on affected and deformed leaves.

Solution: Spray either 5ml of ALONZE 50EC, 30ml of PROFILE 440EC, or 40ml of DEFENDER 25EC per 20 liters of water.

  1. Onion flies: The larvae of onion flies are creamish white maggots that feed on lateral roots, tunnel into the taproot, and sometimes bore into the base of the stem. Infested leaves turn bluish, wilt, and the affected plants may become shriveled and die. The maggots can also be found inside developing onion bulbs, making the plant susceptible to diseases like soft rot.

Solution: Spray either 10ml of KINGCODE ELITE 50EC, 8ml of LEXUS 247SC, or 5g of PRESENTO 200SP per 20 liters of water.

Diseases

  1. Damping off: This soil-borne disease affects seeds and seedlings, leading to the breakdown of plant tissues at the soil level. Infected seedlings may fall over and die, and sometimes the disease can occur even before the seedlings emerge.

Solution: Spray either 50g of CHANCETYL ELITE 800WDG, 40g of MILLIONAIRE 690WDG, or drench with 100g of PYRAMID 700WP per 20 liters of water.

  1. Downy mildew: Infected leaves exhibit a pale-green color that progresses to yellow, followed by collapse and eventual death. Oval-shaped lesions develop on the affected leaves, often becoming prone to other diseases. This disease thrives in cool and humid weather conditions.

Solution: Spray either 25g of GEARLOCK TURBO 250WP, 40g of FORTRESS GOLD 720WP, or 50g of TRINITY GOLD 425WP per 20 liters of water.

  1. Purple blotch: Infection results in small water-soaked lesions with whitish centers, primarily on older leaves. As the infection progresses, these lesions enlarge and develop purplish coloration with light yellow concentric rings on the margins. Leaves may turn yellowish-brown, lose their erectness, and eventually wilt if the disease is left uncontrolled.

Solution: Spray either 50g of COLONIZER 440WP, 10ml of ABSOLUTE 375SC, or 50g of TRINITY GOLD 425WP per 20 liters of water.

  1. Onion rust: Whitish to yellow spots appear on the leaves upon infection, which enlarge over time. These spots transform into orange and eventually develop into black lesions. Heavy infections can result in stunted bulb formation and significant economic losses.

Solution: Spray either 40g of MILLIONAIRE 690WDG, 50g of COLONIZER 440WP, or 10ml of MILESTONE 250SC per 20 liters of water.

  1. Botrytis neck rot: Although originating from the soil, this disease affects stored onions. Infected onions exhibit a watery neck that turns brown and eventually hardens into wizened, papery leaves. In cases of high humidity, a gray mold may develop between the layers of the onion.

Solution: Spray either 15ml of EXEMPO CURVE 250SC, 10ml of EXPLORER 3SL, or 10ml of IPRODE 500SC per 20 liters of water.

  1. Pink rot: This soil-borne disease is easily identifiable as it causes the roots to turn pink or maroon. In severe cases, the roots may die, leading to weakened or stunted plants. The main effect of pink rot disease is a reduction in bulb size.

Solution: Spray either 40g of MILLIONAIRE 690WDG, 15ml of EXEMPO CURVE 250SC, or 40ml of RIMETA GOLD 300SC per 20 liters of water.

  1. Onion smut: This soil-borne disease infects the cotyledon as it emerges from the soil. In most cases, seedlings survive the initial infection, but characteristic black streaks and blisters appear on the leaves and small bulbs later in the growing season as the fungus spreads.

Solution: Spray either 50g of COLONIZER 440WP, 10ml of MILESTONE 250SC, or 15g of RANSOM 600WP per 20 liters of water.

  1. Fusarium basal rot: Initial symptoms include yellowing of leaves and tip dieback. As the disease progresses, the entire plant may collapse, with the roots decaying and detaching upon pulling. The basal plate of the onion typically appears pinkish-brown. If infection occurs late in the season, symptoms may only manifest during storage.

Solution: Drench the soil with 100g of PYRAMID 700WP per 20 liters of water.

  1. Botrytis leaf blight: Early symptoms include greyish-white, oval-shaped spots on the leaves, often surrounded by a silvery white halo with uneven margins. The centers of these spots may become sunken and straw-colored. In severe cases, the leaf tips begin to die back.

Solution: Spray either 15ml of EXEMPO CURVE 250SC, 10ml of IPRODE 500SC, or 10ml of EXPLORER 3SL per 20 liters of water.

  1. White rot: Infection initially causes yellowing and dieback of leaf tips, followed by the collapse of infected leaves. As the infection progresses, bulbs and roots develop white, fluffy molds and soft rots. Tiny black fungal growth can be observed within these molds. Infected bulbs can rot in storage and spread the infection to healthy ones.

Solution: Spray either 40g of MILLIONAIRE 690WDG, 40ml of RIMETA GOLD 300SC, or 15ml of EXEMPO CURVE 250SC per 20 liters of water.

Nutrition & Nutritional Deficiency

To ensure optimal onion production, it is crucial to provide the crop with sufficient nutrients. This can be achieved through both basal and foliar fertilizer applications.

Basal fertilizers are applied at the time of planting and are absorbed by the crop through the roots. They include the following:

  • DAP (Diammonium phosphate): Applied during planting, DAP supplies phosphorus and other essential elements to support the growth of young crops.
  • CAN (Calcium ammonium nitrate), NPK (Nitrogen, phosphorus, and potassium), and UREA: These fertilizers are used for top dressing after the crop has established. They primarily supply nitrogen along with other necessary nutrients.

Since onions have a higher nitrogen requirement, top dressing should ideally be performed at least twice during the growing season.

Foliar fertilizers are applied to the crop’s foliage and absorbed through the leaf tissues. They provide essential nutrients and offer additional benefits. The following are examples of foliar fertilizers:

  • OPTIMIZER: This fertilizer is vital for boosting the crop’s immunity, managing stress, and supplying a range of micronutrients and macronutrients. It can be applied at any growth stage of the crop.
  • GOLDCHANCE SERIES: These fertilizers are specifically formulated to nurture the crop at different stages of growth and development.
  • DIMIPHITE: This fertilizer plays a crucial role in bulb formation.

By utilizing both basal and foliar fertilizers, onion growers can ensure that their crops receive the necessary nutrients for healthy growth and optimal production.

Deficiencies

Nitrogen-deficiency manifests as yellowish-green leaves that are upright, curled, wilted, and dwarfed. The tissue above the bulbs may become soft at maturity.

Correction: Spray either 20ml of LAVENDER per 20 liters of water or 50g of GOLDCHANCE SUPER GROWTH per 20 liters of water.

Phosphorus-deficiency results in slowed plant growth. Leaves appear light green, and bulbs may have few dried outer layers. The tips of older leaves may appear scorched.

Correction: Spray either 20ml of LAVENDER per 20 liters of water, 50g of GOLDCHANCE SUPER START per 20 liters of water, or 10ml of DIMIPHITE per 20 liters of water.

Potassium-deficiency symptoms primarily develop on young leaves due to its mobility within the plant. Leaves become dark green and stand upright, while the tips may experience burning. Bolting, the premature flowering of the plant, may occur. Older leaves turn yellow and necrotic.

Correction: Spray either 10ml of DIMIPHITE per 20 liters of water or 50g of GOLDCHANCE FLOWERS & FRUITS per 20 liters of water.

Iron-deficiency leads to complete yellowing of young leaves. Initially, it manifests as interveinal chlorosis, which then progresses to overall chlorosis, bleaching the entire leaf.

Correction: Spray EURO GOLD PLUS at a rate of 50g per 20 liters of water.

Zinc-deficiency restricts plant growth, while leaves display interveinal necrosis. In the early stages, younger leaves turn yellow and develop pitting on the upper surfaces between the veins. If left unaddressed, these symptoms progress into intense interveinal necrosis, while the main veins remain green.

Correction: Spray ZINC GOLD at a rate of 10ml per 20 liters of water.

Physiological disorders

Bolting refers to the process where onion plants produce flower stalks instead of focusing on bulb formation and growth.

Onion greening occurs when the bulbs are exposed to sunlight, causing the development of chlorophyll.

Sunscald happens when onion bulbs are exposed to direct sunlight during high temperatures, leading to damage and discoloration.

Pinking typically occurs when an onion bulb is bruised, particularly during the harvesting process, resulting in a pinkish discoloration.

Weed Control

Weed management plays a crucial role in ensuring successful onion production. Weeds compete with the crop for essential resources such as nutrients, water, and space, while also serving as hosts for pests and diseases.

To facilitate effective and efficient weed control in onion fields, the following herbicides are recommended:

  • CLAMPDOWN 480SL: This herbicide is applied during land preparation and is effective in eliminating various types of weeds.
  • PREDATOR 340EC: This selective herbicide is applied to the soil immediately after sowing seeds, before seedlings emerge, or before transplanting the seedlings.
  • COMMANDER 240EC: This selective herbicide should be applied 2-3 weeks after transplanting the seedlings.

NOTE: When conducting any foliar spray, it is advisable to mix the product, whether it’s an insecticide, fungicide, foliar feed, or herbicide, with INTEGRA at a rate of 3ml per 20 liters of water. INTEGRA acts as a sticker, spreader, and penetrant, enhancing the efficacy of the product.

Maturity, Harvesting & Post-Harvest Handling

Onions typically require around 4-5 months to reach maturity, although this duration may vary depending on the specific variety and environmental conditions.

The optimal time for harvesting onions is approximately one week after 50% of the leaves have fallen. Signs of maturity include yellowing of the leaves, bulbs turning red, and reaching an appropriate size, among other indicators. It’s important to note that onion bulbs continue to grow even after the leaves have fallen.

Harvesting can be accomplished by either pulling the bulbs manually or by utilizing mechanical harvesters, especially in the case of large-scale farming operations.

Curing Onion

The curing process for onions entails subjecting them to gentle temperatures in a dry and well-ventilated environment. Alternatively, they can be dried in warm shaded areas for a period of time. This process holds significant importance for long-term storage of onions.

During the curing process, the outer skins of the onions dry out and form papery wrappers. Pungent compounds replace the sugars, and the necks at the top of the bulbs come together, creating a seal that prevents moisture and microorganisms from entering.

While in storage, onions are susceptible to sprouting if the storage conditions are unfavorable. Therefore, it is crucial to ensure thorough drying of the onions before placing them into storage.

Storage

Onions are commonly stored using different methods, such as placing them in net bags, crates, pallet boxes, or bulk bins. When utilizing bins or boxes for storage, it is important to provide some ventilation space. Bags of onions are often stored on pallets and should be stacked in a manner that allows for adequate air circulation.

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