Rarely, a gray mycelium develops on the lower leaf surface of infected leaves. Severe infections kill leaves and result in sunburn fruit and weakened plants. The fungus infects weeds and crops in the Solanaceous family and spores are carried by wind to tomato plants. The disease usually occurs late in the season. High relative humidity favors disease development.
Mild temperatures favor infection while higher temperatures hasten the death of infected leaves.
If you see early symptoms, check weekly to monitor the progression of the disease. Multiple, early applications of sulfur are likely to provide control. The other fungicides listed may be more effective than sulfur if applications are well timed. There are no immune tomato varieties in California. Do not alternate or tank mix with fungicides to which resistance has developed in the pathogen population.