On July 26, late blight (caused by the water mold Phytophthora infestans) was formally diagnosed in Wisconsin for the first time in 2017. The late blight sample was of infected tomato fruits from Waukesha County. Late blight attacks both potatoes and tomatoes, and unchecked the disease can rapidly kill plants. Late blight is the disease that caused the Irish potato famine in the 1840’s, resulting in the starvation of approximately 1 million Irish and the mass emigration of approximately another 1 million Irish, many to the US.
Nowadays, late blight can wipe out home garden tomatoes and potatoes, and can have a huge impact on fresh market tomato production. Most importantly in Wisconsin however is late blight’s potential impact on commercial potato production. Wisconsin is a leading potato producer in the US with greater than 60,000 acres in production in 2016.
Symptoms of late blight on potato and tomato leaves and stems typically appear as somewhat large, dark, oily areas, sometimes with a lighter border.
On tomato fruits, the disease often appears as large, leathery areas with somewhat wavy margins and sometimes visible concentric rings. The underside of infected leaves will typically have a fuzzy white-gray appearance, an indication that pathogen is sporulating. Sporulation also occurs on infected fruits, but can be more difficult to see. The disease progresses rapidly and kills plants. The pathogen can eventually also infect potato tubers where it can survive over the winter.
Because of the importance of late blight to Wisconsin agriculture, the PDDC offers free diagnosis of suspect late blight potato and tomato samples. If you see anything that you think is late blight on potato or tomato (or even if you don’t have what you think is late blight on potato or tomato, but want to know what your potato or tomato problem is), send in a sample, invoke the words “late blight” and the diagnosis is free. To submit a sample, place symptomatic leaves, stems, fruits, and/or tubers in sealable plastic bags. DO NOT wrap the sample in wet paper toweling as this can accelerate the deterioration of the tissue and make diagnosis more difficult. Place the bagged sample(s) in a sturdy box with lots of padding and mail the sample to the:
Department of Plant Pathology
University of Wisconsin-Madison
1630 Linden Drive
Madison, WI 53706-1598
Be sure to include complete contact information (i.e., complete mailing address, phone number and email address). If the sample is positive for late blight, PDDC staff will contact you via phone to provide guidance on how to manage the problem. All submitters will receive a written report outlining any disease problems in their samples. Samples positive for late blight will be forwarded to the lab of Dr. Amanda Gevens (the UW-Madison/Extension vegetable pathologist) for genotype testing. There are numerous variants (genotypes) of Phytophthora infestans and knowing which variant(s) [or genotype(s)] are present in Wisconsin can provide critical control information for commercial potato and tomato producers. Certain variants of Phytopthora infestans are resistant to certain fungicides; others are not.
To learn more about late blight and its management in home garden, check out the “Late Blight” University of Wisconsin Garden Facts in the fact sheets section of the PDDC website:
Also, be sure to follow the PDDC on Facebook and Twitter @UWPDDC for updates on plant diseases such as late blight, as well as to learn about PDDC educational events.
For additional resources on late blight (particularly for commercial growers) see the following links: