Online Exclusive: Update on Late Blight in the Northeast

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Online Exclusive: Update on Late Blight in the Northeast

Since the announcement of an early-season outbreak of late blight posted at this site on July 3, this serious disease has been found on more tomatoes and potatoes, it was detected on a weed, and there have reports of lost plantings. But there is some good news: there have been some initial indications of management successes, and preliminary results have been obtained about the pathogen causing this outbreak.

Late blight was found on tomato plants in more garden center retail stores and on tomatoes and potatoes in home gardens and on farms. All tomato plants were removed from many stores, but affected plants continue to be found for sale in some areas. New York state inspectors submitted plants in mid-July that were confirmed to have late blight at Cornell’s diagnostic clinic on Long Island.

Frequent rain and cloudy weather provided favorable conditions for dispersal of pathogen spores and infection. Clouds protect spores from the lethal effects of ultraviolet radiation. Rain deposits spores that are in wind currents onto plants and provides the wet surface they need for infection.

Late blight has been found widely from Pennsylvania and New Jersey north through Maine. In New York, affected plants have been found to date in 35 of the 56 counties and none of the five boroughs of New York City.

A formal survey has not been conducted. Thus, many of the areas where late blight has not been reported reflect the fact that no one has looked rather than the disease has not been found there. In some areas late blight has been found in most of the commercial plantings examined, including high tunnels, ranging from a very low incidence, uncovered through intensive scouting, to common. In other areas, late blight has been found in very few fields that have been scouted.

Unrelenting favorable conditions combined with lack of information on location of potential sources of inoculum in gardens and the fact that this is occurring early in the growing season have understandably made many growers very anxious. Adding to their anxiety is the value of these crops, especially tomatoes, and the expense to grow them. Gardens have not previously been considered an important potential source of inoculum for late blight.

The typical initial source of inoculum for late blight outbreaks in the northeastern region are infested tubers in cull piles, in a field from a previous affected crop, or in seed. Extension specialists and educators working with growers learn about these outbreaks when they start and inform other growers. Electronic communication, in particular a listserv on late blight and the Plant Diagnostic Network, has enabled information about outbreaks to be disseminated widely and immediately. These occurrences in recent years have been relatively rare, compared to other important diseases of these crops.

Late blight has usually been a “potato disease” in the northeast, not only because infested tubers are the primary source, but also because the pathogen strain that has been responsible for recent outbreaks, US-8 genotype, is specialized to potatoes. Tomato appears to be at least slightly more susceptible than potato to the pathogen strain causing the current outbreak based on field observations.

Molecular markers are being used to examine pathogen isolates (individuals) obtained from both garden and farm plants. Based on preliminary results it appears that there is only one strain involved in the outbreak; however, it is important to realize that it is much easier to determine with certainty that isolates are different than the same.

Many growers are likely feeling economically pinched, which they can ill afford under the current economic situation. Late blight is considered difficult to manage under favorable conditions especially when contact fungicides like chlorothalonil are not applied preventively. Some growers and many gardeners have already experienced this.

Best control is obtained with fungicides that target the pathogen and thus are not needed in the absence of this disease. In addition to the added cost to production of using these fungicides is the cost of making the applications. Fortunately there are several targeted fungicides now registered for late blight that are enabling growers to save their crops (e.g. Previcur Flex, Revus, Ranman, Gavel, Curzate, Forum).

Some fungicides, notably Revus Top and copper fungicides approved for organic production, have not been readily available which has added to the challenge of managing late blight. Agricultural chemical manufacturers and distributors cannot afford to produce and stock fungicides for which there is not a predicted need. The current situation with late blight is unprecedented. Federal and state agencies responsible for pesticide registrations promptly responded to low supply of Revus Top and approved a supplemental label for use on tomatoes and potatoes of a related fungicide, Revus.

For more information, photos, and answers to frequently asked questions on late blight, go to http://blogs.cornell.edu/hort/2009/06/26/late-blight-a-serious-disease-killing-tomatoes-and-potatoes-this-year/

McGrath is in the plant pathology department at Cornell University.

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