Update on Late Blight in Pennsylvania

Posted: August 20, 2013

Late blight continues to spread across Pennsylvania and the Northeast region.
Greasy, firm brown lesion characteristic of late blight on an immature tomato fruit. Note the white sporulation of the pathogen on the calyx.

Greasy, firm brown lesion characteristic of late blight on an immature tomato fruit. Note the white sporulation of the pathogen on the calyx.

Late blight continues to spread in counties where it has been previously reported. These counties include Crawford, Allegheny, Indiana, Somerset, Lancaster and Chester. Within the past week, confirmed reports on tomato have expanded to include Berks and Schuylkill counties. Many of the reports have been from home gardens and all the isolates genotyped from the mid-Atlantic and Northeast regions continue to be US23. Late blight is also becoming increasingly more widespread in western and central New York on both potatoes and tomatoes.

Managing diseases like late blight can be challenging especially in high tunnels during harvest due to PHIs. Keep in mind that although many of the same products are registered for both cucurbit downy mildew and late blight, many have different PHIs and they tend to be longer on tomato. On tomato, chlorothalonil, copper and Ranman are the only products with a 0d PHI while Revus Top and Presidio have a 1 and 2d PHI, respectively. Tanos and Curzate have a 3d PHI and Previcur Flex and mancozeb containing products have a 5d PHI.

Scout your fields vigilantly. If using late blight specific fungicides, check corners of fields, areas of the field where you have a more difficult time getting good fungicide coverage with your sprayer, and areas where the foliage tends to remain wet the longest (lower lying shadier areas). This is where late blight symptoms are typically seen first. Protectant fungicides will only protect the crop at the site of application so thorough coverage is critical.

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. The use of late blight specific fungicides will further reduce the development of new lesions and spread of the disease. See the 2013 Commercial Vegetable Production Recommendations for a list of recommended late blight specific products for both tomato and potato. For organic production, copper hydroxide still remains the most effective crop protection tool.

If you suspect late blight on your farm, please contact your local Penn State Extension Office or let me know via email at or by phone at 814-865-7328. We are interested in collecting samples so we can better understand how the pathogen population is changing both within and across growing seasons. 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. 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