Neil Palmer regularly gets down on his hands and knees and crawls among his 1,500 tomato plants in Unity, looking them over with a magnifying glass.

Larry King, of Middlesex, spends $50 to $60 on fungicide for each of his two acres of tomato plants after every rain.

Both local farmers are vigilantly trying to save their crops from Late Blight, a fungus threatening the state's tomato industry and bringing backyard gardeners to their knees as they weep over dead plants.

Late Blight, which has the botanical name Phytophthora infestans, is the same fungus that caused the Irish Potato Famine in the 1840s.

Beth Gugino, a plant pathologist with Penn State University, said the blight has been confirmed in 40 of Pennsylvania's 67 counties.

Barbara Christ, senior associate dean of Penn State's agricultural college, said the impact is going to be "significant," even though commercial growers have tools in their belt not available to backyard gardeners. "They do not have anything that I would call a silver bullet that stops this pathogen."

The first signs of the fungus are gray leaves. A couple of days later, the plants are dead.

"It's nasty," Mr. King said. "You go out to look at your tomato plants and two days later your plant's dead ... black crisp dead."

Mr. King, one of the owners of Harvest Valley Farms, has been spraying fungicide over his 8,000 to 9,000 tomato plants in an effort to save them. Still, there is one row of plants, he said, that juts out a little farther than the others that must not have gotten sprayed as well because they quickly died.

Dr. Christ said the pathogen that causes Late Blight has spread along the entire Eastern Seaboard. "This started in Florida and reaches all the way up to Maine," she said.

And, while it has been confirmed in only 40 counties, she believes it is probably present throughout Pennsylvania.

Much of the spread has been traced to a grower in the South that shipped tomato starter plants to big box home improvement stores along the coast.

"Homeowners were buying these plants not knowing they had something that was infected," Dr. Christ said.

Late Blight is not poisonous, she said, but once tomatoes or potatoes start rotting, they are dangerous to eat.

The cool, damp weather this summer has been perfect for the blight. While farmers know to watch for the disease, home gardeners who do not know about the fungus have seen their crops wiped out.

Mr. Palmer lost 30 percent of his first crop to Late Blight. Since then, he has been inspecting the plants with a magnifying glass every two or three days and spraying them about once a week.

One big buyer of locally grown produce is Eat'n Park, which gets many of its tomatoes from Mr. King's farm.

Kevin O'Connell, spokesman for the restaurant chain based at the Waterfront, said while local farmers have been affected by the pathogen, Eat'n Park has not been hard hit. "At this point it looks like our local farmers have been able to adjust," he said.

Penn State has not been able to tally the cost to farmers of crops lost due to the blight, but Dr. Christ said, "I can tell you, it's going to be significant. We have a significant tomato industry that is used for tomato processing in this state."

Those tomatoes, she said, are grown to be used in tomato sauces. In 2007, the most recent year for which information is available, the state had a $40.6 million tomato harvest.

Potato growers won't know the extent of their losses until fall. The state's crop in 2007 was one-third of what it was in the 1970s, but still there were $22.2 million worth of potatoes grown.

Dr. Christ said half of the potatoes in the state are sold in the produce sections of grocery stores. Others are used for items such as potato chips.

The produce buyers at Giant Eagle so far have not had a problem obtaining fresh, locally grown potatoes or tomatoes, said Dick Roberts, the O'Hara grocer's spokesman.

"Nor do we have any concerns regarding the impact of a blight on the pricing of these products," he said.

Pennsylvania's biggest user of tomatoes, the H. J. Heinz Co., does not expect to be affected by the blight.

Michael Mullen, a spokesman for Heinz, said the company, which is one of the largest buyers of processed tomatoes in the world, purchases tomatoes grown in California and not affected by Late Blight.

Dr. Christ said home gardeners whose tomato plants are killed by Late Blight should pull the plants, bag them in a black plastic bag and leave it in the sun for a few days so the heat will kill the spores.

Affected potato plants, she said, should be treated the same way. But since there are often tubers left in the ground -- and the blight can last through the winter underground -- any "volunteer" potato plants that grow in the spring also should be dug up and destroyed. "We have to be vigilant," she said.

Late Blight, which has the botanical name Phytophthora infestans, is the same fungus that caused the Irish Potato Famine in the 1840s.