Check your garden compulsively and remove infected plants; consider some garden plant replacements, and check out an upcoming late blight webinar.

Once tomato blight strikes your plant, it's a goner. (Photo courtesy of Meg McGrath/Cornell University)

Update: Sign up for a free webinar on July 21; a Cornell plant disease expert will discuss how backyard gardeners can organically control late blight.)

RODALE NEWS, EMMAUS, PA—A Cornell University plant pathologist confirmed Friday that late blight, also referred to as tomato blight—and arguably the most destructive plant disease around—has struck another U.S. region, this time turning up in New England in a Connecticut backyard garden. Another likely case surfaced Friday on Long Island, New York. The disease has also been detected this season in Pennsylvania, Maryland, Kentucky, and Canada. Word of yet another late blight outbreak is sending home gardeners, sustainable farmers, and large-scale commercial growers into panic mode. That's because the funguslike pathogen is highly contagious, and wind can possibly move it between states. Overcast days and high humidity conditions such as we've been experiencing in the East also benefit this funguslike pathogen, which decimated tomato and potato crops in 2009. What's worse, there's little everyday organic gardeners can do to protect their beloved crops from a swift death.

Uh-oh. Not again.

Here's what you need to know—right now—about dealing with late blight, a.k.a tomato blight.

What is late blight?

It's a unique funguslike disease that spreads quickly in tomato and potatoes plants, often killing them. Unlike other plant diseases that can be managed, with late blight, once you notice a plant is infected, it's likely going to face a swift death. Because it's so highly contagious, Meg McGrath, PhD, professor of plant pathology at Cornell University, says education is key. The earlier a diseased plant is spotted and removed, the less likely the disease will spread.

What does it look like?

Telltale signs of tomato blight include brown spots or lesions on the stems, with white fungal growth developing. 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. (See photos.)

Where is it coming from?

McGrath says the first case of tomato blight this year in the U.S. turned up on a farm in Maryland, followed by another in Pennsylvania on tomatoes being grown from seed. Late blight is turning up in fields, greenhouses, high tunnels, and backyard gardens. Scientists believe an infected potato tuber left in the ground, or improperly composted, is responsible, but they haven't been able to find the exact source. "It's like a crime scene and you can't find the criminal," says McGrath. When the infected tuber sprouted this spring, the wind likely took up the spores. The pathogen can survive in the atmosphere during overcast conditions, and falls onto gardens and fields in the form of rain.

Last year's tomato blight outbreak was tied to a nursery that sold seedlings to big-box stores.

What can I do to stop the spread?

Obsessively checking your plants—daily, or once a week at the absolutely least—is a must. In fact, it's a gardener's civic duty. That's because the more people who leave late blight to fester in their gardens, the more spores will spread and infect neighboring plants. "Due to the quantity of spores easily dispersed by wind, a planting with late blight that is not managed—even a small garden—can have devastating impact on other plantings," writes McGrath. "Unlike other diseases, late blight needs to be treated as a 'community disease.' Anyone growing susceptible plants needs to take responsibility and implement a good management program to ensure they don't become a 'typhoid Mary.'"

She suggests gardeners look for and immediately remove any volunteer potato plants (even if they aren't showing signs of disease) that may be sprouting from last year's plantings. If you did not use certified seed potatoes, be even more vigilant about checking for late blight, because those plants are more likely to harbor late blight.

You should also monitor volunteer tomato plants for disease, and remove ones where the disease will be hard to spot, such as those popping up in between pea vines.