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Be On The Watch For Late Blight

Be On The Watch For Late Blight

Early warning: Devastating disease already in Connecticut and Maine.

Late blight has already poked its ugly head up this growing season. One of the earliest reports of the quick and devastating blight to tomatoes and potatoes was confirmed in mid-April in Wisconsin. Since then, it has been confirmed in a Maine greenhouse and a Connecticut greenhouse.

That plus this spring's continuing wet, cool weather prompted New York Ag Commissioner Darrel Aubertine to issue an alert on Wednesday. "The exceptionally cool, damp spring we are experiencing this year heightens our concern," he says. "Growers should take precautions now."

BAD SIGNS: Late blight stem lesions on tomato transplants.

The blight pathogen, Phytophthora infestans, spreads rapidly from plant to plant in wet, cool weather that causes tomato and potato plants, primarily, to wilt and die. In Connecticut, it was on tomatoes grown from farmer-saved seed, and on potatoes grown from organic seed pieces (cultivar 'Australian Crescent'), elaborates Kate Everts, vegetable pathologist at University of Delaware and University of Maryland. In Wisconsin, late blight has also been confirmed on potato seed.

Again, these confirmations weren't widespread. But increased scrutiny of tomato and potato for symptoms is warranted. Symptoms on tomato leaves are lesions that initially appear as light green or grey water soaked areas that expand, says Everts.

Sporulation is white to grey on the under surface of the leaf. Infected leaves die. Petioles and stem lesions are dark brown and irregular.

The blight also can sometimes be found on other crops, weeds and ornamentals, such as petunias, nightshades, and tomatillos. Spores travel through the air, land on plants, and if the weather is sufficiently wet, cause new infections. Once infected, plants may wilt and die within three days.

Organic growers have few approved control measures. For more on identifying late blight and control measures, visit

For potatoes, it is important to plant pathogen-free seed pieces, cautions Beth Gugino, Penn State Extension plant pathologist. Visibly infected seed pieces will have shallow, brownish lesions on the tuber surface and a reddish-brown granular rot under the surface. Only plant certified, healthy seed pieces.

Remember that late blight is a community disease, she adds. If you suspect late blight, please contact your local Cooperative Extension office.

Updates from Penn State will continue to be posted on our Vegetable and Small Fruit Extension Team website ( and audio-messages posted on the 1-800-IPM hotline under the vegetable extension.

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