How to Manage Pests
UC Pest Management Guidelines
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.
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
UC IPM Pest Management Guidelines: Tomato