Late Blight Update: July 9, 2025

As of today, there are no reports of late blight on the East Coast during the 2025 growing season.

Other diseases commonly seen on tomatoes this time of year include Septoria leaf spot and early blight. Early blight also occurs on potatoes. Symptoms of both diseases initially appear on the oldest leaves near the ground. Early blight first appears as irregular lesions that develop concentric black rings, giving the lesion a target-like appearance (see photo). The lesions may or may not have a chlorotic area surrounding the lesion. Septoria leaf spot starts as small, circular, water-soaked spots that develop black to brown borders with a tan to gray center and are speckled with small black fruiting bodies.

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.

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.

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 17, 2025.