Late Blight Update: August 20, 2025

No late blight has been confirmed in Pennsylvania yet this season. The recent dry weather has helped to slow down the spread of disease and keep the risk of infection low.

While potato growers have begun harvesting their crop, tomato growers should continue to monitor their fields for symptoms of late blight. 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.

If you have information to be added to the report, please email Leah Fronk at lxf339@psu.edu. The next report will be posted on September 2, 2025.