Late Blight Update: July 30, 2025

About a week ago, late blight was confirmed in two New York counties, Cattaraugus and Wyoming County. Both potato and tomato are affected. In a tomato field, 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 identified as US-23, which means that it is susceptible to mefenoxam-based fungicides such as Metalaxyl-M or Ridomil Gold.

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

Variable weather patterns occur across Pennsylvania, but with cooler temperatures in the forecast, weather may favor late blight infection in your area. See the most recent Penn State weather update.

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 August 6, 2025.