Garden - Late Blight

Late blight has been found in Delaware. Potatoes and tomatoes are at risk across the state. We are asking homeowners, master gardeners, and professionals that work with homeowners to be on the look for this disease. Late blight moves very easily across long distances and commercial tomato and potato fields are at risk. If you find this disease in gardens, plants should be destroyed.

Late blight was confirmed this morning on potatoes in Kent County, DE. Hopefully this is not the aggressive genotype that caused the last outbreak, the fact that the stems were not infected at this time and that the infections were not deep in the canopy suggests that the source was airborne sporangia (spores) and this is not the US8 that was so aggressive on potatoes. Samples will be checked to see what genotype it is.

Late blight on potato leaf. Photo by Nancy Gregory, Extension Plant Diagnostician, UD.

Underside of potato leaf with late blight. Photo by Nancy Gregory, Extension Plant Diagnostician, UD.

Late blight appears on potato or tomato leaves as pale green, water-soaked spots, often beginning at leaf tips or edges. The circular or irregular leaf lesions are often surrounded by a pale yellowish-green border that merges with healthy tissue. Lesions enlarge rapidly and turn dark brown to purplish-black. During periods of high humidity and leaf wetness, a cottony, white mold growth is usually visible on lower leaf surfaces at the edges of lesions. In dry weather, infected leaf tissues quickly dry up and the white mold growth disappears. Infected areas on stems appear brown to black and entire vines may be killed in a short time when moist weather persists.

Non-comercial growers and home gardeners should also be on the look for late blight. If found in gardens, it is recommended that the plants be destroyed as there are no fungicide products registered for control in the home garden.

Report from Bob Mulrooney, Extension Plant Pathologist, Plant and Soil Sciences Dept., University of Delaware. Late blight symptom description from the Ohio State University factsheet "Late Blight of Potato and Tomato" HYG-3102-95.