The best way to manage the disease is on a preventive basis. Inspect the tips of old onion leaves twice a week for plants with disease symptoms before initiating a fungicide application. It is better not to have multiple or old plantings in the same area because old crops will serve as inoculum for downy mildew in the new plantings. This is especially important when onions are planted for seed production. In the tropics, downy mildew is a problem when onion farms are located in the mountains or cool valleys.
Irrigation Avoid the use of overhead irrigation.
If overhead irrigation is used, apply it early in the day to allow time for the crop to dry. Downy mildew sporulates at night when the leaves are wet. Avoid damp growing conditions and maintain good soil drainage. Seed quality Use certified disease-free seed purchased from reputable seed merchants. Ensure the seed is in the original seed packet.