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Late Blight

University of Minnesota Extension, Stearns County News
August 19, 2015

Source:  Beth Berlin, Extension Educator-Horticulture
University of Minnesota Extension
Stearns, Benton & Morrison Counties

Late Blight
By Beth Berlin, University of Minnesota Extension

ST. CLOUD, Minn. (08/19/15) — What’s happening to your tomatoes or potatoes? At this point in the season there could be a few causes; the first is heat stress, and the second is late blight. Late blight can cause a fast decline to the tomato or potato plant and even impact the produce itself. 

Late blight made its mark in history because of the Irish potato famine in the 1840’s, which caused the death of over a million people and another million to migrate out of the country.  Late blight is caused by the pathogen, Phytophthora infestans and appears on both potatoes and tomatoes; it can occur on other members of the Solanaceae family as well.  It is often referred to as the “community disease” because of its ability to spread from field to field with proper weather conditions. Total crop loss, especially under humid conditions, is not unusual, and therefore identification and proper management is critical.

First symptoms include pale green, water-soaked spots, typically on the tip of the leaves or edges. The lesions are often surrounded by a yellowish border, which then spread rapidly and cause the leaves to turn brown, shrivel, and die.  During periods of high humidity or moist conditions, white fungal growth may also occur on the underside of leaves. Stems may develop dark lesions throughout the plant. The fruit can develop spots that eventually become leathery and brown in color.

Late blight spreads rapidly and can devastate an entire crop within a matter of days. Unlike other plant pathogens, late blight, Phytophthora infestans, does not survive in soil or dead plant debris so crop rotation is not necessary. Instead, infestation is commonly introduced from volunteers, related weeds like nightshade, or introduced from seed potatoes or tomato transplants; viable spores can also blow in from nearby infested crops up to five to ten miles away.  Like many fungal pathogens, the spores thrive in cool, wet weather and become mobile and travel to other plants from wind and water.

Management of the disease is difficult because it spreads so quickly. First, gardeners should select and plant resistant varieties. Control volunteer tomato and potato plants as well as solanaceous weeds like nightshades. Even petunias and potato vines can be carriers. Inspect plants at least once a week and remove any symptomatic plant parts. It is best to remove affected plant tissue in the middle of a sunny day to help prevent spores from dispersing. Put affected plant material in a black plastic bag, cover a pile with a tarp and leave in the hot sun, or dig a hole and bury it. Apply fungicide as a preventative if late blight is found in the area. Once disease is present, it can be impossible to control even if using fungicides. Be sure to read and follow all safety and application directions on the fungicide label.

For more information on late blight visit or



Beth Berlin
Extension Educator, Horticulture
(320) 255-6169
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