It is very easy to grow onion plants. A detailed account of growing practices for onions is given below:

Climate and Soil: Onion is a cool season crop which is subtropical to tropical in its growth habit. High temperature and long photoperiod (i.e. long days) are essential for bulb formation. Seed stalk development is completely dependent on the availability of high temperature and long photoperiod. Temperature is more important than the length of day in seed-stalk development. Onions may be grown in all types of soils. However, the best soil is that is rich in humus and organic matter. High soil acidity needs to be avoided as onion plants are highly sensitive to soil acidity. Optimum pH range is between 5.8 and 6.5.

Propagation and Planting: Seed propagation is practiced in onions. Seedlings are raised in a well-prepared nursery beds. 8 to 10 kg seeds are sown to raise seedlings for one hectare area. Transplanting is done when seedlings are about one or two months old. Spacing depends on cultivar type.

Site Preparation: Main site is prepared by ploughing, levelling and by incorporating organic manures and fertilizers into the top soil. Generally, 10 to 20 tons FYM (farm yard manure) per hectare area is incorporated into the top soil to enhance soil fertility. Ridges are prepared at 3 meter distance and seedlings are transplanted on these ridges at 10-15 centimetres apart.

Manuring and Irrigation: Researches reveal that a yield of 300 kilos of onions removes approximately 150 grams of nitrogen, 420 grams of phosphate and 1300 grams of potash. Half dose of nitrogen and full doses of phosphate and potash fertilizers are applied along with farm yard manure at the time of site preparation. Rest half of the nitrogen is given one month after transplanting. Irrigation frequency depends on climatic conditions and soil moisture level. It is important that soil moisture needs to be kept at optimum level throughout all growth stages of the onion plant. Irrigation needs to be stopped when onion tops start maturing and showing symptoms of wilting.

Weed Management: Regular manual weeding is recommended to keep the plants weed-free during initial stages of plant growth. In addition to this, cultural practices such as crop rotation and choosing weed-free areas for planting may be adopted for weed control. Black plastic mulch is also found to be effective in conserving soil moisture as well as suppressing weed growth.

Disease-Pest Management: Major fungal diseases that affect onion plants are downy mildew, purple blotch, and onion smut. Spraying the plants with a recommended fungicide may be effective in controlling these diseases. Major insect-pests are onion thrips and onion maggots. Spraying the plants with a recommended pesticide/insecticide may be effective in controlling these pests and insects.

A detailed account of disease-pest management practices for onion plants is given below:

Aster Yellow: Aster yellow is a viral disease of onion plants and it is spread by vectors such as onion flies. Hence measures need to be adopted for destroying the vectors/virus carriers. Simultaneously, the affected plants must be removed from the field immediately to avoid further spread of the disease.

Downy Mildew: Downy mildew is a fungal disease caused by Peronospora destructor. It is prevalent in areas having high humidity. Main symptom is violet-coloured furry growth on leaf surfaces and seed stems. Regular spraying of any of the following fungicides such as Cuprous oxide plus rosin soap emulsion, rosin soap with lime-sulphur emulsion, or Bordeaux mixture with cotton seed oil will effectively control downy mildew disease.

Purple Blotch: Purple blotch is another fungal disease that affects onion plants. This is caused by Alternaria porri and the disease affects leaves, seed stems, and lower portions of the plants. The disease begins as small, whitish sunken lesions with purple centers that rapidly enlarge affecting the whole leaves and stems. Eventually, the affected leaves and seed stems fall down from the point of attack and die. Control measure is same as that of downy mildew.

Onion Smut: Onion smut is a fungal disease caused by Urocystis cepulae. Onion smut is prevalent in areas where temperature is below 30OC. Pre-sowing seed treatment with a recommended fungicide may control this disease up to some extent.

Onion Thrips: Onion thrips (Thrip tabaci) may be a big problem while growing onion plants. These are very small, yellow-coloured sucking insects that affect onion plants and other members of the onion family. Silvery-white mottled lesions or blotches appear on the leaves of the affected plants. Eventually, leaf tips turn brown and die. Repeated dusting or spraying with any of the biopesticides such as pyrethrums or chemical sprays with a recommended insecticide can control the insects effectively.

Onion Maggots: Onion Maggots (Hylemya antiqua) are the larvae of grey flies which lay eggs in the soil near the plant base. Eggs hatch in one week and maggots crawl upwards towards the leaves and stems of the plants eating the tender parts of the plants along the way. Regular spraying of Bordeaux mixture may control the onion maggots effectively. The soil around the plant base may be sprayed with a 4:4:50 Bordeaux mixture containing 2% lubricating oil emulsion on regular basis until the maggots are no longer a threat to the plants. Spraying should be repeated every week or every ten days.

Harvesting Stage of Common Onions: Both leaves and bulbs of the common onions are used as vegetables. Tender, young leaves of the plants may be harvested as green onions to be used as a leafy vegetable. Small, immature bulbs along with the tops may be harvested for using as spring onions. A detailed account of various harvesting stages of onion plants is given below:

  1. For using as green onions, spring onions or scallions, young plants may be harvested before bulb formation
  2. For using as summer onions, young plants may be harvested just after bulb formation begins or small, immature bulbs have just formed
  3. For using as pickle onions, young bulbs are harvested so that they can be preserved in vinegar for food purposes
  4. For using as ripe onions, mature bulbs are harvested

Harvesting and Yield: In onion plants, harvest maturity is indicated by the tops drooping just above the bulb while the leaves are still green i.e. 10 to 20% of tops have fallen over. According to UC Davis, “Field-dry” maturity is indicated when bulb neck is completely dry to the touch and not slippery. This stage is typically reached at 5-8% weight loss following harvest. Yield of bulbs is estimated at 10-15 tons/acre or 30-40 tons/hectare.

Post Harvest Management: Freshly harvested onion bulbs are exposed to sunlight for 3-4 weeks in order to be cured before storing them in containers. According to UC Davis, field curing is done when temperatures are at least 24°C (75°F) or exposure for 12 hrs to 30 to 45°C (86 to 113°F) for forced air-curing. After curing, onions are cleaned and graded based on a set of quality parameters. Quality is assessed by the parameters such as mature neck and scales, firmness of bulbs, bulb diameter (bulb size), and degree of pungency. Bulbs should also be free from decay, insect damage, sunscald, greening, sprouting, freezing injury, bruising, and other defects. All decayed, damaged, bulbs with open/bottle necks and injured bulbs are rejected.

Storage: Proper air circulation, and uniformly low temperature and humidity keep the onions in best shape during storage. Mild onions may be stored up to 1 month and pungent onions up to 6 to 9 months at 0°C (32°F) at 65 to 70% RH (relative humidity) with adequate air circulation (1m3/min/m3 of onion). 65 to 70% RH prevents bulb rot during storage but shrivelling rate will be high at this RH.

Storage Loss: Ethylene may encourage bulb sprouting and growth of decay-causing fungi. Shrinkage from sprouting, rotting/decaying and other causes under optimum storage conditions may be up to 15%. Pre-harvest foliage sprays of maleic hydrazide (MH 40) @2500 ppm (parts per million) may be helpful in controlling bulb-sprouting. Irradiation treatment of bulbs also inhibits sprouting and this method is safer than MH sprays.

Physiological & Physical Disorders in Onions: Major physiological and physical disorders found in onions during storage are freezing injury, translucent scales, greening of onions, and ammonia injury. Freezing injury causes soft water-soaked scales in onions. Symptoms of translucent scales resemble that of freezing injury. Ammonia injury causes brownish black blotches on onions. Proper curing of the bulbs followed by cold storage under optimum temperature and humidity conditions will avoid all these physiological disorders in stored onions.

Pathological Disorders: Various pathological disorders that are found in stored onions include different types of mold infestations and fungal rots. A detailed account of pathological disorders of storage onions is given below:

  • Botrytis Neck Rot -Watery-decay at neck area and light gray to gray fungal growth at neck infection and on outer scales. Control: Proper drying and curing of onion
  • Black Mold -It is a fungal infection. Causal organism is Aspergillus niger. Symptom: black discoloration and shrivelling at neck and on outer scales. Control: Low temperature storage following field or handling infestation
  • Blue Mold -It is a fungal infection. Causal organism is Pencillium spp. Symptom: Watery soft rot of neck and outer scales followed by the appearance of green-blue mold fungal spores. Control: Minimize mechanical injury, sunscald, and freezing injury
  • Bacterial Rots/Soft Rot -It is a bacterial infection. Causal organism is Erwinia carotovora subsp. Carotovora. Symptom: Water-soaked, foul-smelling, viscous liquid rot. Control: Harvesting at full maturity; proper drying and curing; minimizing mechanical injury and maintaining proper storage conditions
  • Slippery Skin –Symptom: slippery skin at neck area and on inner scales which have a watery-cooked appearance
  • Sour Skin –Symptom: slimy, yellow-brown decay on inner scales with a sour odor

Seed Production in Onions: If bulbs are left in the soil unharvested, the growing point in the middle of the bulb begins to develop in the next growing season. It is followed by leaf production and soon a long, stout, hollow flowering stalk appears which later produces an inflorescence, a globular umbel of white flowers. Flower production is followed by seed production. Onion seeds are shiny black in colour and triangular in cross section.

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