Late Blight Update: June 3, 2025

As of June 3, 2025, no late blight reports have been made on the East Coast.

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.

Late blight symptoms can develop on leaves, stems, branches, and both green and ripe fruit, and they are very obvious to the naked eye. 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 foliar parts may appear dry and shriveled. Stem lesions are typically brown to almost black in color.

Extension educators have begun visiting farms and scouting for disease. Please email Leah at lxf339@psu.edu to contribute to this report.

The next update will be posted on Wednesday, June 11, 2025.