Late blight is now confirmed in 9 counties in Pennsylvania

Posted: June 22, 2012

This week late blight was confirmed in several additional counties in both commercial production fields as well as home gardens on both tomatoes and potatoes.
Sporulation of the late blight pathogen on the underside of a potato leaf. Photo: Beth K. Gugino.

Sporulation of the late blight pathogen on the underside of a potato leaf. Photo: Beth K. Gugino.

Additional reports of late blight are continuing to roll in. The latest confirmations since June 15th include tomatoes and potatoes a home garden in Centre Co., potatoes on two additional farms in Mifflin Co., on potato in a home garden in Franklin Co. as well as in Berks Co., a commercial potato field in Blair Co. and a commercial tomato field in Synder Co. This brings the total to 9 counties with confirmed late blight in Pennsylvania (Blair, Franklin, Mifflin, Lancaster, Schuylkill, Chester, Centre, Berks and Snyder).

This is the first that late blight has been confirmed in home gardens in Pennsylvania. Once observed in the garden, late blight is very difficult to manage especially if the home gardener prefers not to use any fungicides. It is recommended that symptomatic plants be destroyed and that protectant fungicides containing copper and chlorothalonil be applied weekly to any remaining tomato or potato plants.

US 23 is still the predominant genotype that is being isolated from late blight outbreaks in PA. This was the most common genotype associated with late blight outbreaks in 2011. US23 is characterized as an A1 mating type that can infect both tomatoes and potatoes and is sensitive to mefenoxam (Ridomil). In fields where this genotype has been identified, mefenoxam will help to effectively manage late blight. For resistance management, rotate mefenoxam with another a late blight specific product from another FRAC code tank mixed with a protectant. These include but are not limited to Previcur Flex (FRAC 28), Ranman (FRAC 21), Tanos (FRAC 11+27), Curzate (FRAC 27), etc. Keep in mind that the PHIs for mefenoxam on tomato and potato are 5 and 14 days, respectively.

Please continue to scout your fields and communicate with your local extension office or me if you suspect late blight. I have received numerous phone calls and emails from people with concerns about late blight and I want to continue to encourage that level of communication. If you suspect late blight on your farm, please contact your local county Penn State Extension Office or let me know via email at or by phone at 814-865-7328.

Additional images of late blight on tomatoes and potatoes can be found at the Penn State Extension Vegetable and Small Fruit website under the Vegetable Disease Images link on the homepage at Also for the information regarding where the latest confirmed outbreaks have been reported and to receive email or text alerts about when late blight has been confirmed with a personally defined radius from your location visit