Sep 18, 2009

Late Blight Hits Tomato, Potato Growers Hard

Cool, wet weather, infected plants and a modern distribution chain that can ship plants seemingly everywhere at once all contributed to the explosive outbreak of late blight in tomatoes and potatoes this summer.

The disease has been reported all over the Northeast, the Midwest and beyond, from Maine to Kentucky to Wisconsin. Home gardeners and organic growers have suffered the most, while commercial tomato and potato growers have managed to hold their own. According to those who’ve been following the outbreak, the culprit seems to be infected tomato plants that were sold from garden centers at big box retail stores this spring.

All tomato varieties are susceptible to late blight, symptoms of which include blighting on all aboveground parts of the tomato plant. Lesions on the leaves appear dark and oily, with blackened lesions on the stems that are unique to late blight. Blighting on fruit appears as dark, greasy areas that enlarge rapidly, according to Mary Hausbeck, a plant pathologist with Michigan State University (MSU).

Cornell’s Vegetable MD Online Web site also describes late blight: Caused by the pathogen phytophthora infestans, the disease can infect and destroy the leaves, stems, fruits and tubers of potato and tomato plants. Reproduction occurs via sporangia that are produced from infected plant tissues and disperse to healthy tissues via rain splash or wind currents. Reproduction is most rapid during conditions of high moisture and moderate temperatures – which pretty much describes conditions this year.

Next year shouldn’t be a problem, as long as the weather’s good and the transplants are clean, said Allison Tally, technical brand manager for Syngenta.

Spreading fast

In New York state, late blight reports started to surface in June, about the same time they started to surface in other Northeast states. Home gardeners brought infected tomato plants to the attention of Cornell Cooperative Extension personnel, who investigated the garden departments at big box stores and discovered an epidemic, said John Mishanec, an area vegetable IPM educator for Cornell.

“They found late blight-infected plants in almost every single big box store they went to,” he said.

They also discovered it was a new strain of late blight that hasn’t been seen before. The disease quickly spread from tainted home gardens to commercial tomato and potato fields. New York has seen late blight incidents in the past, but the explosive nature of this summer’s outbreak was extremely unusual, Mishanec said.

Conventional growers took a hit from the outbreak – losing maybe 10 to 20 percent of their crop, Mishanec estimated – but fortunately for them, they had a good repertoire of chemicals they could use to control the disease. The extra spraying added extra cost, but they still had a crop to sell and prices never went down significantly, he said.

It was a different story for the state’s home gardeners and organic growers, who don’t have access to the same kinds of chemical protectants. Mishanec estimated that 80 percent of New York’s homegrown tomatoes were lost this year, while the organic market lost about two-thirds of its tomatoes. The few organic growers who used copper sprays managed to salvage a crop, but many organic farms lost a significant portion, if not all, of their tomatoes, he said.

Commercial potato growers also had to spray more often, but the full impact on this year’s potato crop probably won’t be known until winter, when growers might or might not find infected tubers in storage, Mishanec said.

As bad as the outbreak was, it could have been a lot worse, said Tom Zitter, a plant pathologist with Cornell.

“We kind of dodged a bullet,” he said. “We had some success in an otherwise bleak season.”

Zitter wrote a “report card” on the late blight outbreak in mid-August, shortly after taking a tour of eastern New York farms. He was expecting one dead tomato and potato field after another, but he was pleasantly surprised to find that not everything had been wiped out.

“It was apparent that the disease, although widespread, was not causing total destruction,” he wrote, “and that growers had taken the appropriate steps to minimize their losses.”

The frequent rains, cool temperatures and rash of infected tomato plants could have led to catastrophic losses this year, but this new strain of late blight wasn’t as virulent as past strains have been, according to Zitter.

“Our salvation this summer was that the primary genotype(s) spread throughout the region were apparently not as pathogenic on tomato and potato as we have encountered in other years,” he wrote.

A separate, more virulent strain of late blight attacked potatoes in western New York this summer, he wrote.

The state’s growers still weren’t out of the woods in August, when reports of late blight infection were still coming in. To survive, they had to rely on a tight fungicide program (five- to seven-day schedule for many) with late blight-specific products such as Gavel, Ranman, Curzate, Previcur Flex, Revus and Tanos. Organic growers relied on copper fungicides applied on a three- to four-day schedule (Nu-Cop and Basic Copper 53), Zitter wrote.

He advised potato growers to move their crops as soon as possible to reduce tuber infections in storage. He also advised examining harvested tubers to make sure they’re free of late blight, and removing and destroying any volunteer plants left in the field so they don’t carry the disease into next year’s crop.

The story was similar in other states. In Pennsylvania, late blight was confirmed in at least 40 counties by mid-August, and Beth Gugino, Penn State vegetable Extension pathologist, said it was probably in every other county too, but had yet to be reported.

A number of growers initiated spray programs after the disease was reported in mid-June, and had been spraying ever since. They managed to keep the disease under control, though it was expensive. Organic growers were hit hard, and most home gardeners lost their tomatoes, Gugino said.

In August, Hausbeck, the MSU plant pathologist, wrote that homeowners and organic growers had reported incidents of late blight in Michigan, but that growers who practiced IPM had not reported the disease, since the fungicides they use to fight other diseases also provide adequate protection against late blight.

The first reports of late blight in Wisconsin came in late July, about a week after the first reports from southern Michigan. As of late August, 10 Wisconsin counties had reported late blight on tomatoes and two counties had reported it on potatoes. The outbreak probably spread westward from the Northeast via the aerial dispersal of spores, said Amanda Gevens, a plant pathologist with the University of Wisconsin.

The disease had already hurt commercial and backyard tomato growers in Wisconsin before it made the jump to potatoes. Some potato growers started harvesting early, to get their crops out before their fields became infected, she said.