Unsettled weather = Additional reports of late blight

Posted: August 4, 2012

In the past week or so late blight has been confirmed in three additional counties in Pennsylvania.
Dark brown stem lesions characteristic of late blight on tomato. (Photo: Beth K. Gugino)

Dark brown stem lesions characteristic of late blight on tomato. (Photo: Beth K. Gugino)

The cooler evening temperatures, longer dew periods and unsettled weather have led to several new reports of late blight across Pennsylvania includes Somerset, Columbia and Lehigh counties. This brings the total number of counties confirmed with late blight to 12 with several additional counties having confirmed reports. In some recent cases, tomato fields went from healthy to a complete loss in as few as five days due to the favorable conditions for the pathogen and disease. It is important not to become lax or try to cut corners with your fungicide program. If wet weather is forecasted, consider tightening your spray intervals from 7 to 5-days.

US 23 is still the predominant genotype that is being isolated from late blight outbreaks in PA. 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. As harvest approaches, selecting products that have shorter PHIs will become even more important. Keep in mind that the PHIs for mefenoxam on tomato and potato are 5 and 14 days, respectively.

Remember that late blight is a community disease! If you suspect late blight please contact your local Penn State Cooperative Extension Office, the Penn State Plant Disease Clinic or Beth Gugino at or 814-865-7328. Updates will continue to be posted on this Vegetable and Small Fruit Extension Team website and audio-messages posted on the 1-800-PENN-IPM hotline under the tomato/potato. For the most current map of confirmed late blight outbreaks please visit