Downy Mildew:
The disease is caused by and reported from northern hilly track and plains particularly in high humid locations. The disease is worse in damp conditions and late planting of the crop, application of higher doses of fertilizers and numerous irrigation increased disease severity. Symptoms appear on the surface of leaves or flower stalks as violet growth of fungus, which later becomes pale greenish yellow and finally the leaves or seed stalks collapse.
Control:
To manage the disease effectively, onion bulbs meant for seed crops should be exposed to the 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.
Stemphylium Blight:
The Stemphylium blight is a serious problem in Northern parts of the country especially in the seed crop. This disease is very common on onion leaves and flower stalks. Infection occurs on radial leaves of transplanted seedlings at 3- 4 leaf stage during late March and early April. The symptoms appear as small yellowish to orange flecks or streaks in the middle of the leaves, which soon develop into elongated spindle shaped spots surrounded by pinkish margin. The disease appearing on the inflorescence stalk causes severe damage to the seed crop.
Control:
Field sanitation and collecting and burning of crop residues minimizes the spread of infection. Spraying Mancozeb (0.25%) along with Monocrotophos (0.05%) with sticker triton on appearance of disease at fortnightly intervals controls the disease.
Onion Maggot:
The adults of the insect appear like houseflies. The flies lay their eggs on old leaves or on soil and larva enters into soil and damages the disc portion of onion bulb. Infested plants turn yellowish brown and finally dry up. The affected bulbs rot in storage.
Control:
Regular crop rotation should be followed and Thimet should be applied in the soil before transplanting.
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