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Late Blight Threatens Tomatoes

Late blight, a plant disease that can kill tomato plants, has been found on North Carolina tomatoes earlier in the growing season this year than usual, according to a Cooperative Extension plant pathologist.

Late blight was found several weeks ago on tomatoes in Northampton and Sampson counties and on July 3 in Henderson County, said Kelly Ivors, associate professor of plant pathology and North Carolina Cooperative Extension specialist. Ivors speculated that the plant disease showed up earlier this year because this spring was usually warm.

While commercial tomato growers are generally aware of the presence of late blight and are taking measures to combat the disease, Ivors said home gardeners may also want to be aware of the disease.

Ivors pointed out that prevention, applying a fungicide or other treatment to tomato plants before they are infected, is the best course of action when it comes to late blight. Ivors said plant protection products containing the active ingredients copper or chlorothalonil offer the only effective protection for the home gardener against late blight. Ready-to-use formulations of products containing either of these active ingredients are available at garden centers and stores such as Home Depot or Lowes.

While there are a few tomato varieties that are resistant to late blight, Ivors added, heirloom tomatoes, which many home gardeners like to grow, are not resistant to the disease.

Late blight, which also attacks potatoes, is caused by a fungus-like organism called Phytophthora infestans. The pathogen is best known for causing the devastating Irish potato famine of the 1840s, which killed over a million people, and caused another million to leave the country.

The pathogen likes cool, wet weather. Clouds protect the spores from exposure to UV radiation, while wet conditions allow the spores to infect when they land on leaves. Late blight can be a particular problem in western North Carolina, where nights are cool and fog or heavy dew can help spread the disease.

Ivors said lesions will appear on a plant’s leaves within three to five days of infection, followed by a white cottony growth on the underside of leaves. The cottony growth is evidence that the pathogen in producing spores. Spores may be spread by wind and rain and can be blown several miles, where they may land on other plants and start a new cycle of infection. The disease eventually defoliates and kills the plant.

Ivors said home gardeners who want to see the fruit on their tomato plants turn ripe and red may want to consider protecting those plants now. Once plants show signs of late blight, she added, the best option may be to harvest the fruit, even if it’s green, and learn how to make fried green tomatoes.

Information about late blight, including how to manage the disease, is available online. The website, which is provided by the U.S. Department of Agriculture, provides information on symptoms and reporting outbreaks. The site also tracks the progress of the disease in the U.S.

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