North America and Europe
This fungus generally is not capable of infecting healthy leaf tissue, but it can infect the outer scales of storage onions. Spores of the fungus germinate on onion leaves and produce enzymes that result in superficial flecking. When the fungus grows into the bulb scales, it causes a brown stain on the neck and outer scales. The leaf fleck and bulb stain are usually not regarded as economic problems.
Conditions for Disease Development
Leaf fleck may occur during periods of moist weather with moderate temperatures, 21°C (69°F). Brown stain may decrease during storage because of environmental conditions that do not favor disease development.
The fungus can survive as sclerotia in cull piles, therefore destroying cull piles will reduce this inoculum source. A fungicide spray program to control leaf blight and downy mildew should provide adequate control of brown stain. However, control measures are generally not required for the leaf flecking. Bulb scales that are discolored normally dry out during storage and fall off during handling, leaving the remainder of the bulb salable.