Pennsylvania Growers Urged to Scout for Late Blight Symptoms

By Blake Jackson

Late blight has recently been confirmed in both Pennsylvania and New York, affecting potatoes and tomatoes. In one tomato field, an entire crop of a susceptible variety showed 100% infection, while a resistant variety in the same field displayed significantly less damage.

A separate case was also confirmed on July 17 in Dufferin County, Ontario, where the disease was found on potato plants. In all reported instances, the pathogen has been identified as the US-23 strain. This strain is known to be susceptible to mefenoxam-based fungicides, such as Metalaxyl-M and Ridomil Gold.

Given these developments, growers in Pennsylvania are strongly advised to closely monitor their fields, especially during favorable weather conditions.

Preventive fungicide applications, including products containing chlorothalonil or mancozeb, are recommended on a seven-day rotation. Organic producers can use copper-based fungicides as an effective control measure.

Late blight is a threat to both potato and tomato crops due to its rapid spread and significant economic impact. The disease affects all plant parts leaves, stems, tubers, and fruit.

Initial symptoms typically appear on the upper side of leaves as pale green to brown spots with water-soaked margins. In humid conditions, a downy white fungal growth may form on the undersides of leaves, containing spores that are easily dispersed by wind.

Stems and petioles can develop dark brown to nearly black lesions, leading to the rapid collapse of affected plant parts. Under moist conditions, entire plants can be defoliated within two weeks. In dry weather, infected tissue may appear dry or shriveled.

Spores are most likely to form and spread at temperatures between 65-70°F with relative humidity near 100%. Wind can carry the spores up to 30-40 miles, and they can also spread via dew or splashing rain.

However, their survival drops sharply at humidity levels below 95%, and at 80% RH, spores survive for only about five hours.

Extension educators are actively scouting affected areas. Growers observing symptoms are encouraged to email Leah Fronk at lxf339@psu.edu to support ongoing monitoring efforts.

Photo Credit: istock-martijnvandernat