Late Blight of Tomato and Potato

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The late blight pathogen, Phytophthora infestans, is an important and destructive pathogen on potato and tomato. It also has historical significance as the cause of the Irish potato famine during the 1840s. Wet, cool environmental conditions, such as those prevalent, during the 1840s in Ireland favor development of late blight. During wet and cool weather crop loss from late blight can be rapid and unstoppable if preventative controls have not been used. When late blight is present in a location or is introduced AND environmental conditions are favorable, devastating crop loss can result if preventative control measures are not already in place. A grower who waits until the disease is present in a crop to begin control tactics generally will be fighting a losing battle if environmental conditions are favorable for disease. This factsheet outlines symptoms of late blight on tomato and potato and relevant biology of the pathogen, along with cultural control tactics and preventative fungicide recommendations for use in an integrated pest management program to avoid and manage this devastating disease.