Botrytis leafspot, also called leaf blight, occurs on onions. White sunken spots on leaves are usually signs of infection; spots are small and tend to be oval. They sometimes have a light green halo and may appear water soaked. The epidermis around the spots may be silvery. When numerous spots are present, leaf tips die back and whole leaves may be killed.
Life cycle
Botrytis cinerea spores land on leaf surfaces and, in the presence of moisture, germinate and produce enzymes that kill leaf tissue and provide an entry point for the fungus. The fungus damages the leaf by causing leaf spotting. Leaf surfaces must be wet by dew or rain for long periods (20 hours or more) for leafspot to develop. Optimum temperature for germination of spores is 59°F; optimum temperature for mycelial growth is between about 73 and 77°F.
Solutions
To reduce the level of disease inoculum in the soil, use a 3-year rotation scheme away from onions, garlic, and related crops and destroy volunteer onions during this period.
