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Late blight found on Long Island tomato plants

Established lesions on tomato stems, caused by the

Established lesions on tomato stems, caused by the late blight pathogen. Credit: Cornell Cooperative Extension

For the third year in a row, late blight disease has been detected early in the season on Long Island. This time, it's been found on tomato crops on the South Fork.

Late blight is the disease that was responsible for the great Irish Potato Famine in the 1840s. While it sporadically appears late in the season (August-September) on farms, it's simply dealt with using fungicides. And if it hits late in the season, plants aren't around long enough for entire harvests to be destroyed. But when it appears early, as it has this year and the two years prior, it can threaten to wipe out entire farms. The threat is compounded when the disease occurs in a residential garden because the typical backyard gardener isn't equipped to identify it, so it can run rampant and spread like wildfire, affecting other gardens and, potentially, farms miles away.

Late blight causes white-mold encircled gray spots on leaves and stems that cause the plant to blacken, wilt and die. You might also notice elongated brown lesions on plant stems. Until 2009, the disease had never occurred early in the season on Long Island, and had never been widespread in the United States.

"I'm very concerned that late blight is going to continue to occur every year, especially if we don't all work together on managing it," said Meg McGrath, a plant pathologist with Cornell University working at the Long Island Horticultural Research and Extension Center in Riverhead.

So, what can you do? Inspect your tomato and potato plants for signs of the disease. "I would really appreciate help from gardeners to determine the initial source of this outbreak," McGrath said. "To accomplish I need to know where late blight is occurring. This is really important to do because if we don't figure out the source and understand specifically if and how the pathogen is surviving over winter on LI, then late blight is more likely to keep reoccurring.

McGrath is working with farmers on Long Island to determine if it might have originated on one of their farms, but, she said, gardens also are a possible source. "The outbreak last June on Long Island was in a garden and probably was due to affected potato tubers being used to start new potato plants," she said.

She also is requesting samples of  leaves and stems with late blight symptoms. "This is needed to determine specifically what strain of the pathogen is present," she explained. First, bring the plant or affected plant part to the Suffolk County Cornell Cooperative Extension office at 423 Griffing Ave., Riverhead, for diagnosis. (call 631-727-4126 for help).

If you are unable to deliver your sample, you can send it to the Riverhead office via overnight mail or FedEx, or, as a last resort, email a high-quality digital photo to be used as a first step in the diagnosis. Take photos using the macro setting on your camera (to achieve the best focus) and email to, with "LATE BLIGHT DIAGNOSIS REQUESTED" in the subject line IN ALL CAPS.

Although there are some fungicides available to treat late blight, it's an arduous task, as it needs to be vigilantly and repeatedly applied. My advice is to remove the plant, roots and all, before the next rain to minimize the chances it will spread to other gardens and farms.  Immediately bag the plant tightly in plastic and set in the sun for a few days until the plant dies off completely. Then dispose of in the trash. Do not compost. Do not leave it lying on the ground for later. Here's why:

Late blight is caused by a pathogen that can release millions of spores per plant per day, especially during wet weather. Those spores are carried by wind, potentially long distances, when clouds are present to prevent UV radiation from killing them, until air currents or rain bring them back down, McGrath said. Several years ago, an outbreak on Long Island was suspected to have originated on a farm in Pennsylvania. That's why it's so important to inspect plants and remove any that are affected, disposing of in such a way as not to risk allowing spores to become airborne.

Theoretically, your one infected plant in, say, Floral Park, could wipe out a farm in Riverhead. It's that serious.

If you're not sure your plants are infected -- and chances are you won't be unless you get an expert diagnosis -- place suspect leaves in a sealed zipper-top plastic bag overnight. Late blight spores will develop into a fuzzy mass on the underside of the affected foliage. While this isn't foolproof, it's a good start.

The video at the bottom of this page is from the 2009 outbreak.



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