Late Blight Update: July 23, 2025

Late blight has been confirmed on tomato in Cattaraugus County, New York, on July 18 (see map below). The infection has affected 100% of a susceptible tomato variety, while a resistant variety is affected to a much lesser degree. Late blight was reported on potato on July 17 in Dufferin County, Ontario. In both cases, samples have been submitted for strain ID and results will be shared when available.

With the infection in NY, growers are recommended to monitor their fields diligently, both potato and tomato. In northern PA and areas where weather favors infection, growers should begin to apply preventative fungicides such as cholorthalonil and mancozeb on a 7-day schedule. Copper-containing fungicides are effective preventative controls for organic growers.

The current dominant late blight strain US-23 is still susceptible to mefenoxam-based fungicides. Mefonfexam products such as Metalaxyl-M or Ridomil Gold are labeled for late blight. Additional translaminar fungicides, which can move into and through leaves, include Revus Top, Orondis Opti, Orondis Ultra, Reason, Tanos, Ranman, Curzate, Previcur Flex, Forum, and Presidio. This is not an exhaustive list. Always follow the directions on the label of the product you have on hand.

Variable weather patterns occur across Pennsylvania, but with temperatures cooling recently, this favors late blight infection.

This disease is economically important to tomato and potato. Late blight symptoms can develop on leaves, stems, branches, tubers, and fruit (both green and ripe). Symptoms are very obvious to the naked eye and are often first noticed on the leaves. On leaves, pale green to brown spots appear on the upper surfaces. Leaf spot margins are often pale green or water-soaked. The spots may enlarge rapidly until entire leaflets are killed. In moist conditions, a downy white growth usually develops near the margins of leaf spots on the undersides of leaves. This white growth contains the spores that are easily blown around in the wind. When petioles and stems are affected, portions of plants beyond blight lesions may dry up rapidly and collapse. Lesions can expand rapidly and result in extensive, if not complete, defoliation within 14 days. In dry weather, affected leaf parts may appear dry and shriveled. Stem lesions are typically brown to almost black in color.

With late blight, spore production by the pathogen is favored by temperatures between 65 and 70°F and relative humidity near 100 percent. The spores can travel by wind up to 30 or 40 miles, or over short distances in dew and splashing rain. Survival of the spores is greatly reduced when the relative humidity is below 95 percent; at 80 percent relative humidity, they can survive only 5 hours.

Extension educators are visiting farms and scouting for disease. Please email Leah Fronk at lxf339@psu.edu to contribute to this report. The next report will be posted on July 30, 2025.