Americas, North, East and South Africa, West, South, and East Asia, Europe, Australia, and New Zealand. This fungus is present where temperatures are cool. In the tropics it attacks mainly onions planted in the mountain regions. These areas are pale greenish-yellow to brown in color.
Symptoms often appear first on older leaves.
If weather conditions are moist and temperatures are low, masses of gray to violet fungal spores envelop infected leaves, which become girdled, collapse, and die. Downy mildew seldom kills onion plants, but bulb growth may be reduced. Bulb tissue, especially the neck, may become spongy and the bulb may lack keeping quality. This disease is one of the most destructive of onion seed production world-wide. The downy mildew fungus can rest in perennial varieties and in infected onion bulbs left in the field and in cull piles.