Late Blight on Tomatoes and Potatoes

Update: August 2012

Late blight has been confirmed on both tomatoes and potatoes in Wisconsin. As of Aug. 23, there were confirmed reports from Barron, Adams, Portage, Oneida, Waushara and Marathon counties. All have been the same strain, US-23.
To find the locations of late blight reports in the United States, visit http://www.usablight.org.

Late blight resources for potato and tomato growers

  • Current disease reports and management guidance for Wisconsin are available from Dr. Amanda Gevens at the UW Vegetable Pathology website, http://www.plantpath.wisc.edu/wivegdis/
  • An excellent eOrganic webinar by Dr. Sally Miller of Ohio State University and Dr. Meg McGrath of Cornell University entitled "Late Blight Management On Organic Farms" is available at http://www.extension.org/article/28346. This seminar provides good background on the pathogen, information on occurrence and spread, guidance for scouting and management suggestions for organic growers, but will be educational for growers of any kind.
  • Tomato-Potato Smith Late Blight Risk Map http://uspest.org/risk/tom_pot_map
    This site compiles weather data from all U.S. National Weather Service stations and runs a daily model to determine favorable conditions for disease development.

The problem

Late blight is a disease of tomatoes and potatoes caused by Phytophthora infestans, an oomycete (fungus-like organism). This disease was the biotic cause of the Irish potato famine in the mid-1800s. Under suitable conditions, late blight will spread rapidly through a field, killing plants in a week or less. Infection of fruit or tuber will cause rapid decay, making storage impossible.

The late blight fungus can produce millions of spores on a single diseased plant. Spores can be carried by the wind for considerable distance given suitable weather conditions, causing substantial crop loss for gardeners and commercial growers alike.


On tomato and potato foliage, late blight lesions can occur on leaves, stems or fruit. Lesions usually begin as pale green, then turn brown to black. Lesions can grow quite large, and may destroy entire leaves. Under humid conditions, the lesion margin will show white spore-producing structures, particularly on the underside of leaves. On tomato fruit, brown lesions appear on the top and sides of green fruit.

Differentiating late blight from other common leaf blights (early blight and septoria)

In early blight infections, lesions are generally not larger than ½ inch. Early blight lesions will have tan centers with concentric rings, and often yellow halos around the lesion. Septoria lesions are generally smaller than 1/8 inch in diameter, and will show black spots in the center of the lesion. Septoria generally does not occur on fruit.

Late blight: Note large, gray lesions (turning brown to black) as infection progresses. (photo by Adrian Barta, WI DATCP)
Late blight lesions
Septoria leaf blight: Note small lesions (smaller than 1/8 inch diameter) with dark centers. Generally does not occur on fruit. (photo courtesy of the University of Minnesota Department of Plant Pathology)
Septoria leaf blight

What to do

Infected plants should be destroyed to prevent further spread of the disease.

Small numbers of plants should be pulled and bagged for removal. Large plantings should be tilled down thoroughly. Infected material should not be composted. The Wisconsin Department of Agriculture has authority to regulate late blight.

Managing Cull Potatoes

Commercial potato growers are reminded to handle their cull piles, those potatoes that aren’t market quality, one of three ways:

  • Landspread the culls onto fields then incorporate the culls into the soil.
  • Feed the culls to livestock, making sure the culls are completely eaten.
  • Landfill the culls after gaining permission from the landfill operator.

If a grower does not take the appropriate steps to manage late blight in their fields or to manage their cull piles, the state agriculture department can direct that grower to take corrective action in order to protect Wisconsin’s commercial potato crop.

What to do if you don't have it

Fungicide recommendations are available from UW Extension, at http://www.plantpath.wisc.edu/wivegdis/

Reporting infections

If you are suspicious of tomato OR potato late blight in your home garden or farm, please contact Amanda Gevens at UW (gevens@wisc.edu, 608-890-3072) for assistance with diagnosis.

To help determine the extent of the problem in Wisconsin this year, the Department of Agriculture, Trade and Consumer Protection (DATCP) would like to receive reports of late blight infection from the public and commercial growers. Reports may be sent to: Adrian.barta@wi.gov or call 608-224-4592.

For more information:

Wisconsin Pest Bulletin
Articles in the weekly Wisconsin Pest Bulletin will track spread of the disease. UW Extension will provide updates on treatment options.

Managing Late Blight in Tomatoes - 6 page PDF
University of Wisconsin – Madison, updated August 20, 2009

Tomatoes and Potatoes with Late Blight – are they safe for canning?
Canning guidance from the Pennsylvania State University

Tomatoes and potatoes infected with late blight: Are they safe for eating or preserving? - 2 page PDF
Guidance from the University of Wisconsin-Madison on canning and preserving tomatoes and potatoes that may be infected with late blight.