late blight
Image by Margaret McGrath
Late Blight
late blight on potatoes

Late Blight Update

Destructive Plant Disease - Late Blight - Proliferating Across the Northeast

In recent weeks, Cornell Cooperative Extension educators have been focused on the movement of a devastating plant disease of tomatoes and potatoes (and other nightshade crops), known simply as late blight. Concern lies in just how quickly and easily late blight can spread from garden to garden, from garden to farm, or from farm to farm, even over great distances, with devastating impact.

Caused by an extremely destructive fungal pathogen, Phytophthora infestans, late blight develops and spreads rapidly in host plants in all stages of growth and can easily survive from one season to the next in infected potato tubers left in the ground. Under the right conditions, spores from infected plants can be readily carried by the wind or on storms for miles, or moved from place to place by splashing rain or by animals wandering from an infected garden or field to a garden or field where healthy plants are growing. Under ideal, moist conditions, a single infected tomato or potato plant can transmit the disease to thousands more, in just days.

The late blight pathogen was the cause of the Irish potato famine of the late 1840’s. Most likely, fungal spores were brought to England in the holds of ships coming from North America. Winds then carried the fungus from southern England to the Dublin countryside, where unusually warm, wet weather allowed the infection to thrive and spread quickly to the west.

Millions of Irish peasants, who subsisted solely on potatoes, were left starving and destitute. Those that ate infected potatoes became sick with cholera and typhus. During six years of famine, more than a million people perished. Another 1.5 million migrated to the Americas.

At the time of this writing, late blight infected plants have been confirmed in several New York counties including

Genesee: tomatoes

Livingston: potatoes

Monroe: tomatoes

Oneida: potatoes

Steuben: tomatoes

Tioga: tomatoes and potatoes

Ulster: tomatoes

Wayne: potatoes

Wyoming: potatoes

Yates: tomatoes

Late blight is also reported, in potatoes in Chittenden and Orleans counties in Vermont, although (again, at the time of this writing) samples have not yet been submitted; and in potatoes in Les Jardins-de-Napierville, in Quebec, where samples have been submitted for analysis. Late blight is also confirmed in tomatoes in Essex and Elgin Counties, in Ontario, and samples have been submitted for analysis in Chatham-Kent Division, Ontario. Late blight is confirmed as well, in tomatoes in Erie County, Pennsylvania; in Litchfield County, Connecticut; and in Morris County, New Jersey.

When late blight strikes, entire plantings of home-garden tomatoes and / or potatoes can be unexpectedly lost. And farmers who grow tomatoes or potatoes are put at risk of losing their entire season’s income. To prevent that from happening, Extension educators are asking that we, as gardeners, as neighbors, and as community minded individuals, unite in our efforts to protect both garden crops and the commercial field crops produced by the local farming community.

The Cornell University Department of Horticulture is recommending that gardeners act quickly to protect their home garden tomato and potato plants and to make sure that their plants don’t become a source of spores that could infect other plantings.

If you are growing tomatoes or potatoes in your home garden, you should take the following steps:

- Apply fungicides preventively and regularly

- Examine tomato and potato plants thoroughly, at least once a week, for signs of late blight

- Be prepared to destroy garden tomato and potato plants should you find symptoms of late blight in your garden (place entire plants in plastic bags and dispose of them, into the garbage, immediately)

Cornell Associate professor and plant pathologist, Margaret McGrath, Ph.D., believes selection of resistant varieties is an important strategy for managing late blight, “since late blight is nearly impossible to manage with fungicides under very favorable conditions for the pathogen. McGrath explains, “With resistant varieties, the management practice is in place before late blight starts to develop.”

The ease with which late blight can spread, even over great distances, cannot be overstated. Everyone growing potatoes or tomatoes should be scouting vigilantly for late blight. Keep in mind that the fungicides available for home garden use are recommended as protectants and must be applied before infection occurs, and regularly reapplied.

Please be a part of this effort. Contact your nearest Cornell Cooperative Extension for information about proper fungicide use, timing of fungicide applications and how to monitor for, recognize, and report late blight infection. 

Last updated August 4, 2015