Additional Late Blight Outbreaks Continue to be Confirmed in PA

Posted: June 15, 2012

Late blight has now been confirmed in Blair, Franklin, Mifflin, Lancaster and Schuylkill Counties in Pennsylvania.
Foliar late blight lesions on tomato. Photo: Beth K. Gugino

Foliar late blight lesions on tomato. Photo: Beth K. Gugino

Additional reports of late blight are continuing to roll in. The latest confirmations since June 13th include three commercial potato fields in Schuylkill Co. as well as another commercial field in Mifflin Co. planted with both tomatoes and potatoes. Unconfirmed reports have also been made on tomato and/or potato in Chester, Huntington and Indiana Co. If you have not already done so, it is very important that protectant fungicides should be applied all tomatoes and potato crops in Pennsylvania. If you are located in a county where late blight has been reported including late blight specific fungicides in your fungicide program is also highly recommended. So far there have been no confirmed reports from home gardens although several suspect samples have been examined.

The first late blight reports made on June 4th from a commercial tomato field in Blair Co. and on potato and tomatoes from a commercial farm in Franklin Co. have been genotyped as US23. 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.

 All confirmed outbreaks thus far have been in production fields however, keep in mind that crops grown under high tunnels and other protected structures are not immune from getting late blight. The pathogen does not necessarily require leaf wetness that results from precipitation to cause disease. Extended dew periods that occur with evening cooling and even very high relative humidity can also create conditions that are favorable to infection and disease development. For greenhouse tomatoes both Heritage and Catamaran are registered for suppression of late blight. In fields where late blight has been confirmed, rogueing or burning down the most severely infected plants or portion of the field will reduce the build-up of inoculum and the potential for spread within the field, between fields and between farms. Incorporating the use of late blight specific fungicides will further reduce the development of new lesions and spread of the disease. Products like Tanos (famoxadone + cymoxanil) and Curzate (cymoxanil) have a slight amount of “kick-back” activity and are effective at managing every early stages in the infection process (all of which are invisible to the naked eye).

Copper still remains the most effective tool for organic production. It is important to apply it preventatively before symptoms are observed and since it is a protectant, thorough coverage is also very important. Thorough coverage is important for any type of protectant fungicide. These are only effective where the active ingredient comes in contact with the plant surface.

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