"Late blight has never occurred this early and this widespread in the U.S," said Meg McGrath, associate professor of plant pathology and plant-microbe biology.
One of the most visible early symptoms of the disease is brown spots (lesions) on stems. They begin small and firm, then quickly enlarge, with white fungal growth developing under moist conditions that leads to a soft rot collapsing the stem.
Classic symptoms are large (at least nickel-sized) olive-green to brown spots on leaves with slightly fuzzy white fungal growth on the underside when conditions have been humid (early morning or after rain). Sometimes the border of the spot is yellow or has a water-soaked appearance. Spots begin tiny, irregularly shaped and brown. Firm, brown spots develop on tomato fruit.
McGrath stresses the need to act quickly to protect garden-grown tomato and potato plants and to make sure that plants don't become a source of spores that could infect commercial farms, as late blight spores are easily dispersed by wind.
She recommends that gardeners:
- Examine their tomato and potato plants thoroughly at least once a week for signs of late blight;
- Spray fungicides preventively and regularly, and/or
- Be prepared to destroy plants when late blight starts to become severe.
Petunias, which are closely related to tomatoes and potatoes, can also be infected by late blight and show similar symptoms.
Late blight is very destructive. Uncontrolled, it will kill plants faster than any other disease. And it affects tomato fruit -- especially green ones. Even with fungicide applied every week, there is no guarantee of success, especially if the rainy weather continues. McGrath recommends that gardeners consider growing more of other vegetables this year.
One source of late blight in New York has been traced to tomato plants imported to garden centers from production facilities in the south. If tomatoes were started from seed by a gardener or a farmer in the Northeast, plants are unlikely to be infected, at least initially, she said. If plants were purchased at a garden center and they show signs of late blight, McGrath recommends contacting a local office of Cornell Cooperative Extension or Cornell's Plant Disease Diagnostic Clinic to get confirmation and tell them where you purchased the plants.
"The Committee concluded that the evidence satisfies the criteria contained in the EPA Guidelines for sufficient evidence of carcinogenicity and reaffirmed its classification of chlorothalonil as a Group B2 (probable human carcinogen).
"As currently manufactured, chlorothalonil is contaminated with hexachlorobenzene (HCB) at levels that may accumulate in plants due to repeated applications of chlorothalonil. HCB is classified as a group B2, probable human carcinogen, by the Cancer Assessment Group."
Chlorothalonil; Pesticide Tolerances
[Federal Register: January 24, 1996 (Volume 61, Number 16)]
[Link]