In Joe Lerew's peach orchard, the trees stand leafless and dormant, stripped of their foliage by the cold late-autumn winds and giving no sign of the invisible plague that hides among them.

The plague goes by the name of plum pox, a viral disease of fruit trees that was positively identified in Adams County in September. The disease--incurable and devastating to peaches, plums and other so-called stone fruits--is well known in Europe, Chile and parts of Africa, but it had never been seen in North America.

If the discovery was chilling for orchard owners in Pennsylvania, the nation's sixth-largest peach producer, it was even more so for the state and federal agencies whose job it is to prevent the importation of plant pests and diseases.

"The biggest problem is that we really don't know for sure how it got here," said Lyle Forer, director of Pennsylvania's Bureau of Plant Industry. "It's important to figure out how it may have come in, to identify the pathway and then focus on closing it."

To keep plum pox and other diseases and pests out of the United States, the U.S. Department of Agriculture has rigorous quarantine requirements for imported nursery stock or plant products that might carry unwanted stowaways. Many manufactured products also are subject to inspection for pest infestation before being allowed into the country.

But that Maginot line against imported pests has become more fragile in recent years, as lowered trade barriers have increased the international movement of plant material and created more opportunities for the spread of pests. The sheer volume of imports threatens to overwhelm the inspection capacity of federal and state agencies.

Plum pox slipped in so unobtrusively that it apparently had three years to establish a beachhead in Adams County before anyone realized what it was. "We saw some markings on the fruit three years ago," said Lerew, who farms with two brothers. "We thought it was a nutritional problem of some sort."

The virus, also known as sharka, creates lesions on foliage and fruit that, in mild cases, can resemble a nutritional deficiency or an insect feeding injury. As the disease progresses, the lesions become more serious, making the fruit unsellable. Plum pox also drastically reduces yield, by as much as 80 percent to 100 percent.

When the symptoms intensified the second year, the brothers sent fruit samples to state plant experts to be tested, but the results were inconclusive. "They didn't test for plum pox, because that's not supposed to be a problem here," Lerew said.

This year, the third year after the symptoms were first observed, peach production took a nosedive in the affected groves of trees. In September, the Lerews took samples to an orchardists' gathering at Rutgers University in New Jersey, where a peach expert with European experience recognized the lesions as a sign of plum pox.

When lab tests confirmed the diagnosis, USDA's Animal and Plant Health Inspection Service (APHIS) wasted no time getting to Adams County to assess the extent of the infestation. In Europe, where the disease has infected as many as 100 million trees, stone fruit production is impossible in some regions. In 1994, the disease was verified in Chile, a major world peach producer, and is now considered widespread there.

The virus type identified in Adams County is the D strain of plum pox, the most common strain. The D strain is not as severe or as easily spread as the more virulent M strain, and, unlike the C strain, the D strain does not affect sweet or sour cherries.

The disease can be spread by grafting infected budwood or by using infected grafting tools, but it is more commonly spread by aphids, soft-bodied insects that pick up the virus by feeding on infected foliage or roots. Some strains are also transmitted through seed. The D strain has not been found in stone fruit seed, although it may be transmittable by seed in other plant species that it can infect.

So far, the virus has been identified in 18 blocks of stone fruit trees in four orchards in two townships in northeastern Adams County. Both townships have been placed under quarantine, which forbids the movement of fruit trees or budwood. Scientists worked feverishly through October to collect leaf samples before the trees dropped their foliage, and are now processing 4,000 additional samples from a survey area four miles in diameter.

In an effort to pinpoint the infection source, researchers initially concentrated on checking the sources of nursery stock used to plant the affected orchards, but so far have turned up nothing. "They've done a lot of paper-chasing," said James Lott, who owns one of the affected orchards and also is president of the Pennsylvania Horticulture Association. "They still don't know where it started."

Because the pox affects ornamental trees in the Prunus genus, as well as those grown for fruit production, it is possible that the disease could have come in on a flowering plum or peach tree. The disease also has a large number of alternate hosts, including several common weeds in the nightshade family, white clover and even zinnias. "It could have come from somebody's yard," Lott said. "They might never find the source. The original host plant might be dead and gone by now."

While nursery stock is still the prime suspect, investigators have not ruled out the possibility that the disease was introduced by an amateur gardener, possibly a tourist who had visited an infected area in Europe.

"Somebody could have brought it in hidden in their sock," said Penn State professor Herb Cole, who is the university's liaison with the plum pox researchers. "It could have come in on an innocent-looking, symptomless host, either brought in by an amateur or a legitimate nursery. There are a multitude of questions that aren't answered."

The onset of winter gives researchers a little time to answer those questions. Both the fruit trees and aphids are dormant now, arresting the spread of the virus. But stone fruit trees flower early in the spring--as early as February if the winter is mild--so the respite will be brief.

A group of European experts on plum pox will arrive this week to advise U.S. officials on how best to control the disease, but there seems little doubt that the plan will require destruction of trees and a moratorium on replanting stone fruits in infected areas.

While federal officials focus on the source of the virus and a control plan, Adams County orchardists ponder more immediate economic questions. The county is in the heart of Pennsylvania's fruit-producing region, which produces more than $25 million worth of stone fruit annually. Apples, which are not affected by plum pox, are the main crop in terms of acreage, but peaches and other stone fruits account for 40 percent to 50 percent of revenue for most Adams County growers.

With apple harvest chores behind him, grower Lott ordinarily would be working in his peach orchards now, planning for the annual pruning and tending to the trees' nutritional needs for the coming season. "I'm not doing anything," he said. "I'm not sure at all that those trees will be there next year."