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Late blight found in area potato, tomato samples
UW lab finds pathogen on plants from Adams, Juneau, Sauk counties
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The University of Wisconsin Vegetable Pathology lab confirmed the presence June 28 of the pathogen causing late blight, on leaves from a commercial potato sample submitted from Adams County.

Since then, two additional counties have submitted samples positive for late blight: a potato sample from Juneau County June 29 and a tomato sample from Sauk County July 2.

According to Amanda Gevens, UW plant pathologist, the source of the late blight may have been from infected tomato transplants introduced into the state.  At this time it does not appear that the inoculum came from infected potato volunteers, cull piles, or infected seed potatoes.

Potato late blight lesions from Adams and Juneau counties appeared to be 5-7 days old with a new and initial flush of spore production.  Lesions were few throughout fields and on leaves only – no symptoms on stems.  On the tomato sample from Sauk County, late blight lesions appeared to be several weeks old with large oily black lesions on stems and leaves, and sunken sporulating lesions on ripe and unripe fruit.

Late blight is caused by the fungus-like pathogen Phytophthora infestans.  New strains of the pathogen have been shown to also infect nightshade weeds, but not other solanaceous crop plants such as eggplant, pepper, and tomatillo.

This  is not only of concern to commercial vegetable growers but home gardeners as well. Late blight is capable of quickly wiping out entire potato and/or tomato crops. If undetected and uncontrolled, it could devastate neighboring gardens and commercial vegetable fields. Farmers who grow tomatoes and potatoes are at serious risk of losing their entire income for the season.

UW Extension is currently recommending potato and tomato growers, including home gardeners who do not want to lose their crop, to implement a prevention program using fungicides.

Also, check tomatoes and potatoes closely for symptoms of late blight at least twice weekly. If you suspect late blight on your crop contact your local University of Wisconsin Extension office and send a sample to the plant diagnostic lab for confirmation (no charge for late blight diagnostics at this time).

Severely infected plants cannot be cured and early harvest and plant destruction may need to be considered to limit spore production and risk to area growers.  Destroy infected plants by burying, burning, or putting in plastic bags for disposal. Don’t compost.

Preventative treatments for homeowners are limited to protective fungicides containing chlorothalonil or copper. A home gardener list of fungicides can be found at

Commercial growers should consult the Commercial Vegetable Production in Wisconsin Guide, University of Wisconsin Extension publication A3422 for an extensive list of available treatments.

Leaf symptoms appear as pale green, water-soaked spots that often begin at the leaf edges or tips where water from rain and dew accumulates. Lesions can be circular or irregular and bordered by pale yellow to green blending into healthy tissue. They enlarge rapidly, turning brown to black over time.

When relative humidity is in excess of 90% leaf lesions are often surrounded by cottony white mold on the lower leaf surface. This growth distinguishes late blight from several other foliar diseases of potatoes and tomatoes.

Infected stems and petioles turn brown to black and may also be covered with white masses of sporangia. Stem lesions frequently appear first at the junction between the stem and leaf, or at the cluster of leaves at the top of the stem. Entire vines may be killed very rapidly. A characteristic odor similar to that produced by green tissue after a severe frost can be detected.

Visit the UW-Vegetable Pathology website for late blight photos and links to other late blight information and identification resources.

For assistance in identifying this potentially disastrous late blight disease, contact your local UW-Extension Office.