On Crops: Bulb onions, leeks, chives, garlic and shallots
Worldwide, in humid temperate climates
The tips of onion leaves shrivel and fall over, and pale green to yellowish oval spots form farther down the leaf. Early in the morning, the spots may appear purplish as the fungus matures, or as the colonies are invaded by other fungi. Constant dampness from rainy weather, heavy fog, or crowded plants is needed to trigger an outbreak of onion downy mildew, which reproduces rapidly at 55F (13C). Plants that have been damaged by hail may be infected when spores enter the moist, wounded tissues. Warm, dry weather often curtails this disease.
Onion plants are weakened by downy mildew but seldom killed. Bulbs may be small, with soft outer layers.
Choose a sunny, well-drained site for onions, and control weeds that threaten to crowd the plants. Rotate onions to a fresh site to prevent disease buildup, and water early enough in the day so that onion foliage is dry by afternoon. Gather up and compost onion debris so that the fungus cannot overwinter in onion tissues buried in the soil.
Clip off and compost affected leaves, and weed the bed to admit more sun to the planting. Set aside bulbs from affected plants for short-term consumption, because they tend to rot in storage.