There are no late blight-resistant tomato cultivars. Destroy volunteer tomato and potato plants and nightshade family weeds, which may harbor the fungus. Do not compost rotten, store-bought potatoes. Pull out and destroy diseased plants. Follow the directions on the label.
Infection usually occurs on the lower leaves near the ground, after plants begin to set fruit.
Numerous small, water-soaked spots with dark borders surrounding a light gray center appear on the older leaves. Black specks, which are spore-producing bodies, can be seen in the center of the spots. Severely spotted leaves turn yellow, die and fall off the plant. The fungus can overwinter on crop residue from previous crops, decaying vegetation and some wild hosts related to tomato. Crop rotation and sanitation will reduce the amount of inoculum.