Just as tomatoes are ripening, a fungus threatens to rot the summer crop in the Northeast.

Recent rainy months have created wet, humid fields ideal for late blight, a fungal disease that blackens tomato plants.

The symptoms showed up in Pennsylvania in early July: curled leaves with dark spots sometimes ringed by frosty white spores, and dark greasy spots on the tomato fruits. In particularly bad cases, the entire plant blackens and collapses, emitting a rotten odor.

Samuel Burkholder salvaged the half-dozen rows of tomato plants on his farm north of Fleetwood, Pa., with protective sprays once he discovered spots on the leaves.

"It seems to be controlled, but my plants are not looking as fresh and green anymore. You have the impression that they had some frost on (them)," he said.

Burkholder said his neighbors and other farms near Kutztown, in Berks County, and neighboring Lebanon County have struggled to stay ahead of the damage.

"The general trend is scary," he said. "There's quite a few home gardeners that have it even if they don't know what they saw."

Commercial fungicides are not available for backyard use, but home gardeners can help stop late blight's spread by pulling up diseased plants and disposing of them in the garbage, experts said.

Penn State University plant pathology professor Alan MacNab has tracked the spread of late blight this season in Maine, New York, Pennsylvania, North Carolina, Michigan and eastern Canada.

In an Aug. 2 memo to Pennsylvania horticultural agents, MacNab wrote: "This means late blight will almost certainly appear in many additional fields. ... Remember that late blight spores can infect plants after being wind-blown 50 miles or so."

Lancaster County horticulture extension agent Tim Elkner estimates half the county's 212 farms that grow processing tomatoes - from small Amish and Mennonite plots to 40-acre commercial growers - have been hit with late blight.

"Pick a spot. We're pretty much covered up," he said, including his backyard garden in Marietta, Pa.

"This is a disease which, under ideal conditions, it's very aggressive and this year we've had ideal conditions," Elkner said. "We've had excessive rains late in the day or early evenings so everything stays wet all night, plus cooler temperatures."

Agricultural experts say late blight spreads to tomatoes from potatoes, from the same fungus that caused the mid-19th century Irish Potato Famine.

Tomato plants are susceptible to many fungal and bacterial diseases, said Mary Hausbeck, a professor of plant pathology at Michigan State University.

"People need to remember that these micro-organisms start out much like seeds ... and those seeds also need moisture to get started and infect the plants and cause spots," Hausbeck said. "Anything homeowners can do to keep those plants dry will protect the plant."

New Jersey confirmed its first case of tomato blight last week in Cumberland County. Wesley Kline, the county's extension agricultural agent, said all commercial tomato and potato growers in the state have been advised to spray with protective fungicides.

Farmers in the state saw their production of processing tomatoes drop by 30 percent last year after a bad bout of late blight, Kline said.