Plant only sound, blemish free tubers to avoid late blight and other tuber-borne diseases. Don't allow any discarded tubers to sprout and possibly produce late blight spores. The first appearance of lesions commonly occurs after periods of wet weather. Black lesions appear within 3-7 days of infection of leaves. Under humid conditions, delicate, whitish fungal spore producing structures are produced at the edge of the lesion, particularly on the underside of the leaf.
Lesions turn brown when they dry up, and are often surrounded by a halo of gray-green tissue.
Once lesions dry up, the white spore masses will not be visible. To help identify late blight if outdoor conditions are not humid enough for spores to be produced, you can place suspect leaves or stems in a closed container with a damp paper towel. Check the leaves after about 12 hours to see if the delicate, white fungal material is growing from the tissue at the edge of the lesion. On stems, late blight causes brown, greasy looking lesions that frequently appear first at the junction between the stem and leaf, or at the cluster of leaves at the top of the stem. On tomato fruit, late blight causes a firm, dark, greasy looking lesion from which the fungal spore producing structures emerge under humid conditions.