Garden Detective

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

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Late blight alert! Tomatoes, potatoes at risk

Late blight-affected leaves found on an infected tomato

Late blight-affected leaves found on an infected tomato plant in Riverhead. (July 25, 2013) (Credit: Meg McGrath)

To those who thought the threat of late blight this year was over and we tomato and potato growers were out of the woods, so to speak, unfortunately that isn't the case.

Word came down from the Cornell Cooperative Extension this morning that symptoms of the devastating pathogen were discovered yesterday on tomato plants in Riverhead. Because the source of infection is spores that are easily dispersed by wind, it's likely they'll be making their way across Long Island, as they did during the past four years.

Cornell plant pathologist Meg McGrath, who has been tracking the disease since it first surfaced in the area, reports "Infection occurred at least seven days ago, maybe as long as 12 days, which is right in the heat wave." Documentation indicates "the new strains of the pathogen tolerate hot weather better than the old ones," she added.

The past outbreaks all began in June, carried to the area on infected plants from growers in other areas, but McGrath believes the source of this year's outbreak is different: "I think this outbreak is more likely due to pathogen spores being moved long distances in wind." 

Unfortunately, McGrath says, this week's cool, overcast, rainy weather "is perfect for the late blight pathogen to be dispersed and cause more infections."

What does this mean to you? Once again, we all need to be vigilant in looking for symptoms and reporting them as soon as possible. "Gardeners not only need to look thoroughly at their plants for symptoms now, but they need to continue to do so, especially about a week from now," McGrath advises.

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.

"Hopefully gardeners are learning that late blight is unlike other diseases because it is so contagious and so destructive, and so needs to be responded to," McGrath says. "All of us growing tomatoes (but especially farmers) need to know how widespread late blight is, and we all need to manage it when it affects our plants."

On the plus side, growers have developed resistant tomato varieties, which McGrath reports did well in her  trials last year. She has another trial underway now to evaluate some new varieties including one (Iron Lady) that is expected to be close to immune.

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..

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