Many farmers say that tomatoes are their most important cash crop and that the blight will be devastating. “Tomatoes get me out of debt every year,” said Kira Kinney, an owner of Evolutionary Organics in New Paltz, N.Y., who has late blight on potatoes and tomatoes and expects that most of the crop will be destroyed. “I go into the season with credit card debt and I come out O.K.,” she said. “That’s how I cover my annual costs for the whole farm.”

On July 23, Billiam van Roestenberg said that 11 of the 12 growers who participate in the weekly farmers’ market he runs in New Paltz had already seen late blight in their fields that was likely to ruin their crops. The next day, the 12th farmer  Mr. van Roestenberg himself  found the disease on his own tomatoes.

Late blight, which caused the Irish potato famine in the mid-19th century, thrives in damp, windy weather. Its symptoms include white powdery spores, brown spots on leaves and open lesions, each of which can produce hundreds of thousands of infectious spores. Burning, spraying and deeply burying infected plants are options for farmers; home gardeners should pull plants out at the first sign of the disease. Rather than composting them, the plants should be sealed in plastic bags and thrown away.

Every state in the Northeast and mid-Atlantic has confirmed recent cases of late blight, which normally does not appear in the region until August, if at all. The source of the outbreak is being investigated by pathologists. Home gardens likely helped spread the infection: Lowe’s, Home Depot, Kmart and Wal-Mart all sold tomato seedlings with late blight in their garden centers from April to June. All are offering refunds or credits to gardeners who must destroy their plants.

But there is no similar recourse for farmers. Even those who have not lost a crop to blight are suffering financially because of it. To ward off the infection, which has been sweeping through farms in her area, Ms. Hepworth has been spraying all her plants with a covering of fixed copper, an approved organic fungicide that creates a physical barrier preventing spores from reaching the plant. Because copper, unlike synthetic fungicides, washes off in heavy rain and must be carefully reapplied, “It costs me $1,000 every time it rains,” she said.

Dale Mohler, an agricultural meteorologist at AccuWeather.com, said that low temperatures in June and July broke records across the Northeast and that rainfall is running 50 to 100 percent higher than normal around the region. Mr. Mohler, who said he lost his own home-grown tomato plants to late blight, said August isn’t likely to bring the sustained hot weather  about 10 days with temperatures above 85 and dry conditions at night  that could stop the continued spread of late blight.

Like other growers, David Hambleton, a farmer in Dutchess County, N.Y., whose crop is shared by about 250 members of the Sisters Hill Farm community supported agriculture program, is concerned that members who do not receive the vine-ripe juicy summer tomatoes they look forward to will not pay $500 to $700 for a share next year. “Last year was a bumper crop, one of the best ever,” he said. “This year, we’ll have to ask our members to participate in local agriculture in a more realistic way.”

Farmers who do not practice organics, like Bill Maxwell of Changewater, N.J., are using pesticide sprays to protect their tomatoes, but still must worry about blight, weather and the state of the crop, which is running about a month late. “I have huge, beautiful cauliflowers, but I’m not going to make a lot of money on that in July,” he said. “People want their tomatoes.”