UPDATE: See our story on tomato blight in the 2010 growing season!
RODALE NEWS, EMMAUS, PA—Anxious home gardeners have been frequenting Meg McGrath’s office lately, tomato plant specimens cradled close to their bodies like unwell children. And more often than not this season, McGrath, a PhD and associate professor of plant pathology at Cornell University in Ithaca, New York, has been forced to delivery the dreaded news: Yes, it’s late blight. “The look on their faces, it’s like I just told them one of their children has a contagious, lethal disease,” she says.
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And as far as plant diseases go, the potato- and tomato-attacking late blight, a funguslike pathogen, is one of the worst. The disease outbreak hitting the northeastern United States this season is devastating farmers relying on the cash crop, and depressing home gardeners who look forward to the season’s sweet, succulent, vine-ripened tomatoes. McGrath says this outbreak is of historic proportions—our country has never seen one as early in the season or an outbreak as widespread as this. And yes, this is the same type of blight that caused the Irish potato famine in the mid-1800s. “To me, as I look at what’s happening, I’d say this is the worst agricultural crisis I’ve seen in my 21-year career,” says McGrath. Read on for answers to your tomato blight questions. |
Are my tomatoes at risk?
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Not every tomato plant in the affected area is doomed, but the chances of not being affected are really pretty low, explains McGrath. Scientists are trying to trace the root of the problem, and there’s a widespread hypothesis that infected plants came from a nursery that supplies plants to many of the big-box stores, such as Wal-Mart and Home Depot. But even if you didn’t buy your plants from these retailers, your plants are still very much at risk. |
Photo courtesy of Meg McGrath/Cornell University
Why is this happening?
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The unusually wet and cool month of June in the Northeast created perfect conditions for infected plants to produce spores of the blight. On windy days, those spores are carried from fields and gardens and up into the atmosphere. Typically, UV radiation from the sun will kill the spores, but the chronically cloudy conditions this season have allowed the spores to spread. McGrath says while they are in the atmosphere, the spores can travel impressive distances, possibly between states. When it rains, the precipitation scrubs the spores from the sky and they fall back to gardens and fields, where they sicken other tomato and potato plants. |
How do I know if my plants have it?
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Signs of late blight include brown spots, or lesions, on the stems, with white fungal growth developing. If the stem is OK but you start to see at least nickel-size olive-green or brown spots on leaves with some white fungal growth underneath in the early morning or after rain, your plant has most likely been struck by the disease. Sometimes the spot is surrounded by yellow or looks water-soaked, explains McGrath. |
Photo courtesy of Meg McGrath/Cornell University
What should I do with an affected plant?
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Short answer: If your stems are struck, pull the plant. The chances of it surviving are next to none. To avoid spreading the pathogen, double-bag the diseased pieces, let the bag sit in the sun for a day, and then put the bags out for garbage pickup. |
Photo courtesy of Meg McGrath/Cornell University
Can’t I spray the plants to kill the disease?
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Spraying probably won’t save diseased plants, and chlorothalonil, the fungicide used to deal with late blight, is classified by the U.S. Environmental Protection Agency as a “probable human carcinogen.” It’s available to household gardeners in the form of Bravo. But don’t use it. Lab tests have found it causes kidney and liver damage, anemia, miscarriages, damage to cells’ genetic material, and kidney and stomach cancers. It’s also toxic to fish, frogs, shrimp, beneficial microorganisms in your soil, and earthworms. The pesticide has been linked to colony collapse disorder, which has been wiping out honeybees all over the country. This season’s tomatoes aren’t worth the health risks to you, your family, or the environment. |
Organic growers sometimes use copper sulfate to deal with late blight, but this intervention isn’t completely safe for humans or wildlife, either. Plus, given the size of the current outbreak, you’ll be hard-pressed to find it in the store because manufacturers didn’t make enough to deal with an outbreak of this magnitude.
Isn’t there anything I can do?
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“Late blight is a very destructive and difficult-to-manage disease,” explains McGrath. “A spot can form within four days of when a spore lands on the plant, and a day later be producing spores that can be wind-dispersed to healthy plant tissue, resulting in more spots within a few days.” That said, there’s a small chance that you may be able to save your plant if you live in an area where few outbreaks have been reported, if just a few of your plants are affected, if conditions are expected to be hot with no rain for a prolonged period, and if you can regularly cut off plant tissue affected by the late blight. Remember to dispose of the diseased pieces as described above. |
Photo courtesy of Meg McGrath/Cornell University
But…what about my tomatoes?
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There’s no way to get around it. This outbreak really stinks, especially for commercial farmers. If you’re a home gardener who has to pull your plants, you can still harvest the unripe tomatoes. If the fruit harbors brown spots, just cut them off and make your favorite tomato soup, or fried green tomatoes, with your early harvest. The fungus is not toxic to humans. There’s still time to plant leafy vegetables like lettuce, kale, or spinach in the spot where you pulled your tomato plants—they won’t be affected by the blight. |
What about next year?
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The pathogen needs tomato, potato, or bittersweet nightshade weeds to thrive, so your soil won’t harbor the disease next spring. You’ll be able to plant tomatoes (or potatoes) again, and hopefully the weather patterns won’t support another blight outbreak. At this point, it’s an open question whether commercial greenhouse operations ship out infected plants every year. But we think this is a good reason to buy your seedlings from a small, local nursery. |