Destruction of cull piles and rotation out of onions for at least 2-3 years are important to help prevent the buildup of inoculum and reduce the likelihood of severe epidemics. Because these cultural practices are only partially effective, and no blast-resistant varieties are available, onion growers must rely upon repeated application of protective fungicides for acceptable disease control. Some farmers in Michigan use a weather-based computer program to schedule fungicide applications, thereby eliminating unnecessary sprays. Purple Blotch Introduction Purple blotch is a disease of onion and related crops caused by Alternaria porri , a fungus affecting tops and bulbs. The disease is prevalent throughout the onion growing regions of the Midwest.
Symptoms first appear as small tan spots on leaves.
The lesions become sunken and rapidly expand rapidly up and down the leaf. Numerous leaf lesions contribute to collapse of the entire top. Wounds occurring at or shortly before harvest provide sites for Alternaria infection of onion bulbs. Disease Cycle . The pathogen survives midwestern winters on infected crop debris.