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Late Blight Confirmed on Tomato in Lancaster Co., PA

Posted: June 13, 2012

To-date late blight has been confirmed on five farms in four counties and more unconfirmed outbreaks are suspected.
Whitish-gray sporulation characteristic of late blight on the underside of a tomato leaf. Photo: Beth K. Gugino

Whitish-gray sporulation characteristic of late blight on the underside of a tomato leaf. Photo: Beth K. Gugino

Today late blight was confirmed in Lancaster Co. in a commercial tomato field. This is the first confirmed report from Lancaster. Symptoms were most severe on a couple of plants that have been rogued from the field. Foliar lesions were observed on several nearby plants. A sample is being submitted to Cornell for genotyping.

So to-date, late blight has been confirmed on potato and/or tomato on a total of five farms stretched across four counties (Blair, Franklin, Mifflin, and Lancaster) in Pennsylvania. Currently it is suspected, but not yet confirmed, in two additional counties and there are several suspected outbreaks in counties that have been confirmed with outbreaks of late blight. 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.

We are still awaiting confirmation on the genotypes of the potato and tomato samples submitted to Cornell. This is important because some of the more recent genotypes of late blight have been sensitive to mefenoxam, the active ingredient in Ridomil. This provides conventional growers will fields infected with a mefenoxam sensitive genotype an additional tool in their toolbox. It is also important because it will help us better understand how the pathogen population is changing and therefore make necessary adjustments in our management programs.

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.

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). Applications of these products need to be follow-up with an application a fungicide from another FRAC code group. See the 2012 Commercial Vegetable Production Recommendations for a list of recommended late blight specific products for both tomato and potato.

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 bkgugino@psu.edu 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 http://extension.psu.edu/vegetable-fruit. 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 http://usablight.org.