Late blight—Phytophthora infestans
Phytophthora infestans has a wide host range, including tomato, potato, pepper, and eggplant. On leaves, late blight lesions typically first appear as irregular, small pale to dark green water-soaked spots that are surrounded by a zone of yellowish tissue. Lesions may expand rapidly and become brown to purplish black. White sporulation of the fungus may be observed at the periphery of lesions, principally on the underside of leaves. On stems and petioles, lesions are brown to black and may also support sporulation of the fungus. Fruit discoloration usually begins on the upper side of the fruit. Affected fruit remain firm. Tubers develop a firm brown decay.
Life cycle
Late blight occurs commonly in coastal environments and sporadically elsewhere. The fungus inoculum can originate from seed tubers, cull piles, volunteers, closely related weed hosts such as nightshade, and adjacent plantings of potatoes or tomatoes that are affected. Late blight can develop and spread rapidly if inoculum is present and conditions are conducive. High humidity and average temperatures in the range of 50 to 78°F favor the disease. Air circulation to facilitate the drying of foliage each day is important. Overhead sprinklers can favor late blight.
SolutionsUse certified plants and tubers. When late blight has developed on foliage and fruit or tubers are at a risk of infection, make sure that vines have been completely dead for 2 – 3 weeks before harvest as the fungus does not survive very long in dead foliage. Avoid sprinkler irrigation. Destroy all tomato, potato, eggplant, and pepper debris after harvest.
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 Late blight lesion on potato stem and petiole
 Late blight symptoms on potato leaves
 Brown decay in potato tubers
 Late blight infection on tomato leaves
 Tomato fruit discoloration
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