For potatoes, these lesions can serve as pathways for other tuber diseases like bacterial soft rot to enter, so late blight symptoms can sometimes be obscured by symptoms of other diseases. On tomato fruit, Phytophthora infestans causes a firm, dark, greasy looking lesion where, under humid conditions, the spore producing structures emerge.
Infected potatoes have shallow, brownish or purplish lesions on the surface of the tuber. Symptoms on tomato leaves and stems are similar to those on potato. Lesions turn brown when they dry up and the white spore masses will not be visible. When it is humid, whitish growth (pathogen spores) is produced at the edge of the lesion, particularly on the underside of the leaf. Black lesions appear within 3-7 days of infection of leaves. Late blight lesions can occur on both leaves and stems, and usually occurs after periods of wet weather. infestans infects the foliage of the plants. Unlike the other 60 Phytophthora species that produce soil-borne, root-rotting diseases, Late blight infects foliage, stems, potato tubers and tomato fruits. Late Blight, Phytophthora infestans, is an eukaryotic, water mold (Oomycete) which caused the 1845 Irish and 1846 Highland potato famines.
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