Colletotrichum gloeosporiodes or Colletotrichum circinans
Symptoms : The symptoms appear initially on the leaves as water soaked pale yellow spots, which spreads lengthwise covering 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.
(Fusarium oxysporum f.sp.ceapae)
Symptoms : Yellowing of leaves, stunted growth, root and dry rot of stem plates.
Development : Moderate temperature of 22-28ºC favours.
Management: Soil solarization Seed treatment with Thiram or Carbendazim (0.3%). Foliar and soil application of Carbendazim or Benomyl or Thiophanate methyl (0.1%) Growing tolerant varieties.
Symptoms : The disease is common in onions stored in hot climates where the temperature ranges between 30- 450C. It is characterized by the black powdery mass of spores that appear on the exterior of the scales. The black spore masses are also seen on inner scales. It reduces the market value of the bulbs
Control: For effective control of die disease, left for drying in the field for two days. These bulbs should be further dried in shade for 10-15 days before storage. Care should be taken to avoid injury to the bulbs during post harvest handling. The crops should be sprayed with Carbendazim (0.2%) 10-15 days before harvesting
(Fusarium oxysporum f.sp.cepae; Pythium sp.; Sclerotium rolfsii and S. cepivorum and Colletotrichum sp.)
Symptoms : The disease is more prevalent during kharif season and causes about 60-75% damage. High soil, moisture and moderate temperature along with high humidity especially in the rainy season leads to the development of the disease. Two types of symptoms are observed
Pre-emergence damping-off: The pre-emergence damping off results in seed and seedling rot before these emerge out of the soil.
Post-emergence damping-off: The pathogen attacks the collar region of seedlings on the surface of soil. The collar portion rots and ultimately the seedlings collapse and die
Control: Healthy seed should be selected for sowing. The seed should be treated with Thiram @ 2g/kg of seed before sowing. Continuous raising of nursery in the same plot should be avoided. The topsoil of nursery should be treated with Thiram @ 5g/m2 area of the soil and nursery should be drenched with the same chemical @ 2g/litre of water at fortnightly interval. Soil solarization by spreading 250 gauge polythene sheet over the bed for 30 days before sowing and application of bio-control agent Trichoderma viride in soil @ 1.2kg/ha is also found effective to control damping-off to considerable extent.
Symptoms : The disease is caused by and reported from northern hilly track and plains particularly in high humid locations. The disease is worst in damp conditions and late planting of the crop, application of higher doses of fertilizers and numerous irrigation increased disease severity.
Development : Symptoms appear on the surface of leaves or flower stalk as violet growth of fungus, which later becomes pale greenish yellow and finally the leaves or seed stalks collapse.
Control: For managing the disease effectively, onion bulbs meant for seed crop should be exposed to sun for 12 days to destroy the fungus. Spraying with Zineb (0.2%), Karathane (0.1%) or Tridemorph (0.1%) also gives good control of the disease.
Symptoms : Initially small water soaked lesions on leaves or on seed stalks that rapidly develop into white centers Zonation and brown to purple color of spots.
Development : Moderate temperature, high humidity and moisture favour the disease development.
Management: Reduce plant density and promote good field drainage to reduce hours of leaf wetness. Application of fungicides such as Chlorothalonil (0.2%) or Propineb (0.2%) or Mancozeb (0.2%) at fortnightly interval from the onset of the disease. Use Resistant Varieties.
Symptoms : It occurs on white onion varieties and reduces the market value of the bulbs. The disease is characterized by small dark green to black spots, which appear on the outer scales. Control: Thorough curing of the bulb after harvesting and storing the bulbs in wellventilated rooms can control the disease. Colletotrichum is mainly a problem with white onions. Rings of black fruiting bodies can be seen on the dried onions.
Development : This disease mainly arises towards the end of the growing season and continues to develop in storage. The fungus can remain in the soil for many years on plant debris. During warm, humid weather the spores are released and can infect the outer scale. After infection, new fruiting bodies form quite quickly, producing more spores.
Control: After harvesting the onions should be quickly dried until the outer scale is thoroughly dry. Ensure good drainage of the soil and that the crop is free of diseases. Apply crop rotation and consider cultivating yellow or red onions instead of white in problematic regions
Stemphylium Leaf Blight (Stemphylium vesicarium)
Symptoms : Small, light yellow to brown and water-soaked lesions develop on leaves and blighted leaves Lesions usually turn light brown to tan at the center and later dark olive brown to black as the spores develop.
Development : Long periods of warm wet conditions Lesions generally occur on the side of the leaf facing the prevailing wind
Management: Application of fungicides such as Chlorothalonil (0.2%) or Propineb (0.2%) or Mancozeb (0.2%) at fortnighty interval.
(Iris yellow spot virus)
Symptoms : Eye shaped or diamond shaped spots will appear on scapes; further it coalesce and cause severe damage to foliageEye shaped or diamond shaped spots will appear on scapes; further it coalesce and cause severe damage to foliage.
Development : Thrips transmits the virus under natural conditions.
Management: Similar to thrips management in other crops