Infected fruit frequently drops. The fungus survives on infected debris in the soil, on seed, on volunteer tomato plants and other solanaceous hosts, such as Irish potato, eggplant, and black nightshade. Use pathogen-free seed and do not set diseased plants in the field. Use crop rotations, eradicate weeds and volunteer tomato plants, fertilize properly, and keep the plants growing vigorously. Follow the directions on the label.
Late blight is especially damaging during cool, wet weather.
The fungus can affect all plant parts. Young leaf lesions are small and appear as dark, water-soaked spots. These leaf spots will quickly enlarge and a white mold will appear at the margins of the affected area on the lower surface of leaves. Infected tomato fruits develop shiny, dark or olive-colored lesions, which may cover large areas. Fungal spores are spread between plants and gardens by rain and wind.