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. The spores mature early in the morning and are dispersed during the day. They remain viable for about 4 days. After the fungus is established, it completes its life cycle in 11 to 15 days.
New spores infect new plants or leaves.
As the upper portion of the onion leaf is killed, the fungus can infect the next lower part of the leaf. The entire leaf may thus become infected and die. During favorable environmental conditions the infection may result in a severe epidemic. During dry weather, the spores usually disappear and the number of lesions declines. However, the disease cycle recommences when wet, cool weather recurs.