Late blight, caused by the fungus Phytophthora infestans, rarely occurs in Iowa but can devastate tomato plantings during periods of cool, rainy weather. Late blight may infect either young (upper) or old (lower) leaves. It first appears as water-soaked areas that enlarge rapidly, forming irregular, greenish black blotches,
giving the plant a frost-damaged appearance. The undersides of the leaves often show a downy white growth in moist weather. Infection of green or ripe fruit produces large, irregularly shaped brown blotches.
Infected fruits rapidly deteriorate into foul-smelling masses. Late blight usually appears in mid- or late August during persistent cool, wet weather, or when cool night temperatures cause frequent heavy dews. P. infestans causes similar symptoms on potatoes and can spread from potatoes to tomatoes.
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