Late Blight Present in Upstate N.Y.

Published online: Sep 03, 2020 Articles, Fungicide
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Source: Observer

The Cornell University Cooperative Extension Vegetable Program is warning farmers about the presence of late blight in the Chautauqua County, N.Y.

Disease spots are often dark gray to brown in color and may or may not have a ring of pale green tissue around them. They are often irregular in shape and size, and often become as large as a quarter. Leaf spots will often have small fuzzy white spores on the underside of the leaf in wet and humid conditions. Symptoms progress quickly and are often first found in areas where airflow is poor, such as near weedy patches, hedgerows, in low-spots, or where the canopy is dense. Late blight will put dark brown to black smears on plant stems. Tomato fruit may also develop large, greasy-looking, brown, gray, or black smears on the upper part of the fruit. Late blight does not resemble yellowing leaves with lots of small black specks that are worse lower in the canopy.

Potatoes will exhibit similar foliar and stem lesions. In potatoes the disease can have a darker presentation and more quickly kill foliage. Late blight can and will infect potato tubers. Infected volunteer potato tubers carry the disease over from one year to the next. Because of this, any potatoes which are close to maturity should now be mown off and the vines thoroughly killed to prevent late blight infection. Tubers should be left in the ground for 3 weeks. Tubers cannot be infected if there are no vines above ground, and the waiting period allows infected tubers to start showing disease symptoms.

Late blight is much more easily prevented by regular (weekly) applications of chlorothalonil, which goes by product names like Bravo, Echo, and other generics. Chlorothalonil must be applied before disease presence, and will slow the on-set of symptom development. Cornell Cooperative Extension officials said it is likely that many farms in the area have already been exposed. Growers should have at least one effective treatment material on-hand.

Late blight is difficult to stop once it establishes in a canopy, and is best treated quickly. Examples of effective chemistries include Ridomil, Gavel, Ranman, Orondis, Revus, Zing and Zampro. Materials can be used inside high tunnels so long as there is not a greenhouse use prohibition statement on the label. Other options are listed in the Cornell Vegetable Recommendations.

Those who suspect they have late blight should call Cornell Cooperative Extension call immediately. Extension officials need to track the disease to help protect other farms by knowing where it has moved to and helping farmers keep their outbreak from sporulating. Those who suspect they have late blight should call Emily Reynolds, CCE chautauqua executive director, at (716) 664-9502, ext. 201, and leave a message including the farmer’s name, farm location and species affected.

More details, photos and a Reference chart of fungicides used for late blight control, are available on the Cornell Cooperative Extension website at