How to Manage Pests
UC Pest Management Guidelines
Powdery Mildew on Greenhouse and Coastal Tomatoes
Pathogens: Oidium neolycopersici
(Reviewed 12/13, updated 12/13)
In this Guideline:
Symptoms of powdery mildew are limited to leaves. Symptoms initially appear as light green to yellow blotches or spots that range from 0.125 to 0.5 inch (3–12 mm) in diameter on the upper surface of the leaf. A white, powdery growth of the fungal mycelia and spores is obvious on the top of leaves. As spots coalesce, the leaf tissue dies. The entire leaf eventually turns brown and shrivels but remains attached to the stem.
Comments on the Disease
In California, powdery mildew caused by O. neolycopersici is limited to greenhouses and fields close to the coast. Conidia are easily windborne and are carried long distances. The conidia land on leaves where they germinate and enter the leaf stomata. The fungus grows at moderate to cool temperatures. Little moisture is required for the fungus to establish itself on a plant. There is experimental evidence that the pathogen has a wide host range and probably survives on other hosts or volunteer tomato plants from season to season.
This powdery mildew is generally not severe in coastal fields and control measures are usually not warranted. Greenhouse-grown tomatoes, however, can suffer to the point of severe economical damage. Registered fungicides, such as sulfur, may be required to control the disease in the greenhouse. Begin applications when the disease first appears.
Organically Acceptable Methods
Sulfur sprays are acceptable for use on organically certified produce.
UC IPM Pest Management Guidelines: Tomato
R. M. Davis, Plant Pathology, UC Davis