Treating late blight in tomatoes | Living the Country Life
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Treating late blight in tomatoes

Heirloom tomatoes are hit the hardest
Heirloom varieties are most susceptible to late blight

Late blight, a disease that can kill tomato plants, is showing up earlier than usual because spring 2012 was unusually warm, says Kelly Ivors, associate professor of plant pathology and North Carolina Cooperative Extension specialist. Ivors says 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. 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.

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