tomato blight

If you see signs of the disease on your potato or tomato plants, remove the plant from your garden, bag it, and send it to the landfill.

Brown patches on the leaves are a symptom of tomato blight. Photo courtesy of Meg McGrath/Cornell University

For more on tomato blight, see our latest update.

RODALE NEWS, EMMAUS, PA—Anything but the tomatoes! Unfortunately for home gardeners and farmers alike—including some of us here at Rodale.com and our Organic Gardening colleagues—the fungus-like pathogen Phytophthora infestans, which causes the infamous late blight, commonly called tomato blight, is starting to appear in parts of the country. And, as before, it's threatening gardeners' most prized possessions—their juicy, homegrown tomatoes. "Unfortunately, I think it's a matter of time before it's widespread," says Meg McGrath, PhD, associate professor of plant pathology at Cornell University in Ithaca, New York. "Hopefully, people will be more knowledgeable of the disease this year, realizing it's a community disease. If you end up with it, you have to take care of the problem, you can't let it fester in your garden."

THE DETAILS: Maryland reported the first outbreak this growing season, with Pennsylvania, Kentucky, and Canada following suit. The disease is cropping up in all types of settings, from backyard gardens to fields and high tunnels on farms. McGrath says plant pathologists have not been able to pinpoint the source of each outbreak this spring, but notes that the disease needs a living host and can overwinter in potatoes left in the soil or in storage. (Late blight affects potatoes too, and caused the Irish potato famine in the mid-19th century.)

Photo courtesy of Meg McGrath/Cornell University

Telltale signs of blight include brown spots or lesions on the stems, with white fungal growth developing.

Photo courtesy of Meg McGrath/Cornell University

If a stem is OK, but you start to see nickel-size or bigger olive-green or brown patches on a plant's leaves, with some white fungal growth underneath, your plant has most likely been struck by the disease. The brown areas are likely to first appear in the early morning or after rain. Sometimes the spot is surrounded by yellow, or looks water-soaked, explains McGrath.

Last year's tomato blight outbreak got rolling after gardeners in the Northeast, not knowing this disease including its symptoms because it typically occurs very rarely, bought infected plants at garden centers and didn't know this is one disease requiring a "community response" (notify and destroy). Tomato blight spores on infected plants are easily swept up in wind currents and carried high into the atmosphere, where they can potentially travel for long distances, even between states. The spores survive up there in ideal weather conditions (rainy, or even just humid and overcast days), and then fall into gardens and onto farm fields with the next rain.

WHAT IT MEANS: This is bad news for this year's tomato crop. What's worse, McGrath says if tomato blight sticks around for a few more years, it could very well become endemic in this area. That's bad news especially for organic growers, who nix the use of toxic fungicides. And while some copper products are approved for organic use, they aren't benign to soil or human health, either. "For organic, copper is the best choice," she says. "But people should make sure they understand that just because it's an organic product, it isn't as safe as water."