Garden Detective

Jessica Damiano's award-winning garden blog gets to the root of things.

Late blight advice for home gardeners

Numerous initial symptoms of late blight are visible

Numerous initial symptoms of late blight are visible on these tomato leaves. (Credit: University of Maine Cooperative Extension)

Late blight has been identified on Long Island for the fifth straight year, and once again, we all need to be vigilant in looking for symptoms and reporting them as soon as possible.

Late blight, the disease responsible for the great Irish Potato Famine in the 1840s, reared its ugly head on Long Island in 2009, and has been back every year since. The disease is caused by a pathogen that can release millions of spores per plant per day, especially during wet weather. Those spores are carried long distances by wind until air currents or rain bring them down to infect more plants, potentially dozens of miles away. Theoretically, spores from one infected plant in Riverhead could destroy an entire crop on a farm in Nassau. 

The disease causes elongated brown lesions on stems and large grayish-green to brown spots on leaves that cause the plant to blacken, wilt and die. White mold full of spores encircles spots visible on the undersides of leaves. If late blight is detected, plants should immediately be bagged tightly in plastic and set in the sun for a few days until they die, then disposed of only in the trash, never composted or left on the ground in piles, as spores will continue to form and disperse until the plant dries up.

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.

If possible, bring affected leaves, transported in a sealed plastic bag, to one of the three Long Island CCE locations for diagnosis: In Nassau, the office is at East Meadow Farm, 832 Merrick Ave., East Meadow, and in Suffolk, samples are being accepted at both the clinic at 423 Griffing Ave., Riverhead and the CCE office at Bayard Cutting Arboretum, 440 Montauk Hwy., Great River. Doing so will benefit you because if your plant does not have late blight, you'll know you won't have to yank them out. And it will help us all to help the CCE track where the blight is surfacing.

On the plus side, growers have developed resistant tomato varieties, and trials are underway to evaluate some new varieties including one (Iron Lady) that is expected to be close to immune.

Here's a map that tracks late blight outbreaks around the country this year: usablight.org.

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