How to Manage Pests
UC Pest Management Guidelines
Onion and Garlic
Pathogen: Botrytis cinerea
(Reviewed 1/07, updated 6/08, pesticides updated 6/16)
In this Guideline:
Botrytis leafspot occurs on onions. White sunken spots on leaves are usually the first sign of infection; spots are small—0.06 inch (0.5 mm) up to 0.25 inch (6 mm) long—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.
COMMENTS ON THE DISEASE
Botrytis cinerea spores land on leaf surfaces and, in the presence of moisture, germinate and produce enzymes that kill leaf tissue. The fungus damages the leaf by causing leaf spotting. Leaf surfaces must be wet by dew or rain for long periods (20 or more hours) for leafspot to develop. Optimum temperature for germination of spores is 59°F; optimum temperature for mycelial growth is in the mid-70s (°F).
Isolate seed fields from fresh market or processing onion fields as bulbs are a major source of spore inoculum. To reduce the level of inoculum in the soil, use a 3-year rotation scheme away from Allium crops and destroy volunteer and cull onions during this period.
Monitor fields and apply a treatment at the first evidence of leaf spotting.
UC IPM Pest Management Guidelines: Onion and Garlic
R. M. Davis, Plant Pathology, UC Davis
Acknowledgment for contributions to Diseases:F. F. Laemmlen, UC Cooperative Extension, Santa Barbara and San Luis Obispo counties
R. E. Voss, Vegetable Crops, UC Davis