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Late Blight Continues to Spread Across Pennsylvania

Posted: August 29, 2012

With the cooler night temperatures, extended dew periods and recent rains moving across Pennsylvania, late blight is continuing to spread both in commercial fields as well as home gardens.
Classic foliar late blight lesions are greasy brown with a pale green margin. On the underside of the leaf will be white powdery sporulation (Photo: Beth K. Gugino).

Classic foliar late blight lesions are greasy brown with a pale green margin. On the underside of the leaf will be white powdery sporulation (Photo: Beth K. Gugino).

Numerous outbreaks of late blight continue to be reported from on tomatoes and potatoes in both commercial production fields and home gardens. To-date late blight has been confirmed on tomato and/or potato in at least 21 counties including Franklin, Blair, Mifflin, Lancaster, Schuylkill, Chester, Centre, Berks, Snyder, Columbia, Lehigh, Somerset, Cumberland, York, Northumberland, Cambria, Fulton, Westmoreland, Perry, Wayne, Montour and Lackawanna Counties. So far, the samples collected from late July/August that have been submitted to Dr. Bill Fry’s lab at Cornell for genotyping have been genotyped as US23 or have been very similar to US23. This is similar to the samples submitted for genotyping in June before the several weeks of hot and dry weather. Conditions over the past several weeks have been very favorable for the spread of late blight. In order to reduce the amount of inoculum that can infect and cause disease to your neighbors’ crops or garden it is important destroy the plants as soon as possible if you no longer to or are able to manage the disease. If fact, I had to destroy a couple of research trials this past week for this reason.  The more quickly the plant tissue is killed, the more quickly the pathogen will be killed. Consider killing the plants with a herbicide, cutting and tarping the plants under plastic, or using a weed burner to kill the foliage. Harvest as many of the fruit as possible and ripen off the vine. Discard any fruit that develop symptoms. If symptoms develop post-harvest, this indicates that the fruit were infected while in the field. Since the pathogen is an obligate pathogen (requires living plant tissue to survive) it will not survive overwinter unless in infected potato tubers so there is no need to “treat” the soil in preparation for next years’ crops.

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 bkgugino@psu.edu or 814-865-7328. Thanks to everyone who has been in communication with me and sent samples for genotyping. We are still interested in collecting samples especially from counties where late blight has not been previously reported. For the most current map of confirmed late blight outbreaks please visit http://usablight.org.