Late Blight Update: August 5, 2025

No late blight has been confirmed in Pennsylvania yet this season. With the dry weather forecasted, the risk of infection is low.

Western New York on the Pennsylvania border has confirmed late blight in 3 counties (Cattaraugus, Steuben, Wyoming). Both tomato and potato are affected. However, scouting in Erie County potato fields last week did not detect disease. Growers are encouraged to monitor their fields regularly, both potato and tomato, for disease symptoms. Before the next storms arrive, apply preventative fungicides such as cholorthalonil and mancozeb. Copper-containing fungicides are effective preventative controls for organic growers.

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. If you suspect late blight on your farm, contact your closest Extension Educator, or email Leah Fronk at lxf339@psu.edu. The next report will be posted on August 13, 2025.