Cutworms:
The larvae of this insect are seen in nursery beds and newly transplanted onion fields. The tender plants are found damped at ground level during the night Young larvae feed gregariously on foliage but later segregate and enter into soil. Young larvae are yellowish grey and later on become brown, greasy to touch and coil when disturbed. They cut the seedlings at ground level during night and hide during day.
Control:
Soil application of Carbofuran (1kg a.i./ha) at the time of planting is recommended.
Chlorpyriphos (5ml/litre of water) also gives good control of this pest.
Onion Thrips:
Thrips are the most important insect pests on onions. Adults are yellowish to yellowish brown with narrow wings. Eggs are laid in clusters in leaf tissues. Numerous nymphs and adults are observed between leaf sheaths and stems lacerating the epidermis of leaves and sucking the exuding cell sap. The affected leaves show silvery blotches which later turn into brownish colour. The leaves get distorted from tips downwards and the plant ultimately wilts and dries away. Seedlings are killed due to heavy infestation and the plant growth is retarded. The shape and size of bulbs as well as yield is affected by thrips.
Control:
Soil application of Phorate or Carbofuran granules (1kg a.i./ha) is also recommended. Alternate spraying of 0.07% Endosulfan (2ml/litre of water), or 0.07% Nuvacron (2ml/litre of water) or 0.01% Cypermethrin (1 ml/litre of water) along with a sticker (Triton or Sandovit) at fortnightly intervals controls the insect.
Anthracnose:
The symptoms appear initially on the leaves as water soaked pale yellow spots, which spreads lengthwise covering the entire leaf blade. The affected leaves shrivel and droop down.
Control:
Since the pathogen survives on crop debris, sanitation and destruction of infected crop debris helps in reducing the disease. Mancozeb (0.25%), Carbendazim (0.1%) or Thiophanate Methyl (0.1%) as foliar spray is effective against the disease.
Onion Yellow Dwarf:
This is a viral disease caused by onion yellow dwarf virus. It is transmitted mechanically as well as by insect vectors. The symptoms of the disease are severe stunting of the plants, dwarfing and twisting of the flower stalk. The affected leaves and stems change their normal green colour to various shades of yellow and leaves tend to flatten and crinkle and as a result bend over.
Control:
Removal and destruction of the diseased plants checks the spread of the disease. Healthy bulbs should be used for seed production. Spraying of Malathion (0.1%) or Metasystox (0.1%) to control the vectors checks further spread of the disease.
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