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Late blight of tomato and potato

Quick facts

  • Late blight is a potentially devastating disease of tomato and potato, infecting leaves, stems, tomato fruit, and potato tubers.
  • The disease spreads quickly in fields and can result in total crop failure if untreated.
  • Late blight does not occur every year in Minnesota. 
  • Late blight of potato was responsible for the Irish potato famine of the late 1840s.

How to tell late blight apart from other issues

Leaf infections are large brown blotches with a green gray edge. Photo: Michelle Grabowski.
An amorphous, brown leaf spot caused by late blight on potato. Photo: Howard F. Schwartz, Colorado State University, Bugwood.org
Infected fruit have a dry brown rot. Photo: Michelle Grabowski.
Potato tuber infected by late blight are shrunken, discolored, and soft. Photo:Scott Bauer, USDA Agricultural Research Service, Bugwood.org
Late blight infects leaves, stems and fruit
  • Leaves have large, dark brown blotches with a green gray edge; not confined by major leaf veins.
  • Infections progress through leaflets and petioles, resulting in large sections of dry brown foliage.
  • Stem infections are firm and dark brown with a rounded edge.
  • In cool, wet weather, entire fields turn brown and wilted as if hit by frost.
  • In tomatoes, firm, dark brown, circular spots grow to cover large parts of fruits. Spots may become mushy as secondary bacteria invade.
  • In high humidity, thin powdery white fungal growth appears on infected leaves, tomato fruit and stems.
  • Infected potato tubers become discolored (anywhere from brown to red to purple), and infected by secondary soft rot bacteria


  • Late blight (Phytophthora infestans) is a water mold.
  • Late blight favors cool (60°F to 70°F), damp conditions. Prolonged hot dry days can halt pathogen spread.
  • Late blight doesn’t appear in Minnesota every year. 
  • The most common routes of introduction each season are infected potato seed tubers, infected tomato transplants shipped in from other regions, or windblown sporangia (asexual spores) from the south that then infect fields and circulate locally.
  • P. infestans can overwinter in Minnesota if protected in potato cull piles. Overwintering in a tomato production system is unlikely but infected tomato fruits may give rise to infected volunteer seedlings the following season.
  • Under cool, wet conditions, P. infestans can infect and produce thousands of sporangia per lesion in less than five days. These sporangia easily become air-borne, resulting in prolific spread of the pathogen.
  • There are many different strains of P. infestans. These are called clonal lineages and designated by a number code (i.e. US-23). Many clonal lineages affect both tomato and potato, but some lineages are specific to one host or the other.
  • The host range is typically limited to potato and tomato, but hairy nightshade (Solanum physalifolium) is a closely related weed that can readily become infected and may contribute to disease spread. Under ideal conditions, such as a greenhouse, petunia also may become infected.

Managing Late Blight in the Home Garden


Managing Late Blight on Farms


Marissa Schuh, Horticulture IPM Extension Educator, Anna Johnson; Michelle Grabowski, and Angela Orshinsky

Reviewed in 2021

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