Researchers, farmers and agricultural specialists are engaged in a vigorous drive to eradicate a virus threatening stone fruit orchards. While the effort has been confined to three Pennsylvania counties, its success could allow fruit growers to sleep a little easier.

Plum Pox is a virus that spreads in one of two ways. It's carried by aphids, which spread the disease while "tasting" a plant to see if it's edible.

The second method is by humans when propagating, a common process by which
farmers graft part of one tree onto another, in order to establish a new variety. If the graft is infected, the virus is unintentionally transferred to a new host.

The virus is a problem because it starts by making the fruit unmarketable and eventually renders the tree barren.

No one knows how the virus spread to Pennsylvania although odds are it was through human propagation. While the disease has been widespread in Europe for more than 60 years, it's only shown up in North America during the past few years after first being found in Chile in 1992. Adams County, Pennsylvania reported an infected plant in 1999 and a nursery in Ontario, Canada did the same in 2000.

The disease can be difficult to detect. In fact, one part of the tree can be healthy, while another part is infected. So if an infected area isn't tested, it's possible for a diseased tree or limb to go undiscovered. Symptoms of the disease may take three years or more to show in an infected tree, although testing can usually discover it earlier. This means an infected tree could be propagated (via an infected limb being grafted to a healthy tree) without any way of telling the disease exists.

"Fruit trees are always budded," said John Halbrendt, an associate professor of plant pathology at Penn State University's Fruit Research and Extension Center. "If you don't know you're dealing with infected wood, then it may be infected early on and introduce the virus into trees that are going to be sold and shipped throughout the country.

"Fortunately, it appears we found the virus before it got into any propagation materials, but it means we need to re-examine our certification process because plum pox was not on the list because it wasn't believed to be in the United States."

Halbrendt cited Europe as an example of how bad it can get when things go wrong. More than 100 million trees are believed to be infected with the disease there.

"The Europeans have had it for years," he said. "In the early days they didn't know much about it or how it spread and didn't know what precautions to take. A lot of stone fruits are very susceptible and they didn't know there are different strains of virus and they differ in how easily they're transmitted."

The people and organizations trying to deal with this outbreak know they have a daunting task. The U.S. and Pennsylvania departments of agriculture are conducting an aggressive effort against the disease said Halbrendt.

"Pennsylvania has taken a very aggressive stand toward eradication," he said. "So even if one tree is found, the entire orchard is destroyed. There's also a 500-meter radius cleared, even if it's a neighbors orchard, so we are rapidly destroying the reservoirs of the virus."

The Plum Pox eradication program has already destroyed more than 1,400 acres of stone fruit trees in the four affected counties (Adams, Franklin, Cumberland and York) according to information provided by Nancy Richwine, a plant pathologist and survey coordinator with the Pennsylvania Department of Agriculture. There's about 120 peach or nectarine plantings per acre on average.

"We're optimistic that there's not a pocket of disease somewhere that we haven't found it," Richwine said. "That's not to say that there's not somewhere where we might find an occasional tree that's infected.

"The way we try to prevent the spread of the disease by propagation is through the quarantine. The way we prevent infections by aphids is by removing the infected trees. Since aphids generally don't travel that far under their own speed, the majority of their spread is more localized somewhere between 500 or a 1000 meters."

Combined the federal and state survey efforts in Pennsylvania have cost more than $1.5 million and the federal government is spending additional money for a more national study. This doesn't include the money farmers are receiving as compensation for destroying their trees.

"If Plum Pox is detected in a commercial orchard," Richwine said, "that grower is required to remove and destroy the neighboring trees. They are eligible for up to $1000 to recover the cost to remove the, plus they're also eligible for indemnity to cover loss of income from lost production.

"This is all based on what crop they're growing (some crops bring in more revenue than others) it also depends on age of tree (these trees have a productive lifespan of about 20 years, so a 15-year-old tree is valued lower than a six-year-old tree for example) and a couple of other factors. So there's a sliding scale of reimbursement per acre. The average has been about $12,000 an acre."

While no one's saying the problem's been solved, Halbrendt says it appears the aggressive effort may eventually lead to the eradication of the disease.

"I'm optimistic that it's going to happen," he said. "If you asked me two years ago, I wouldn't have been as optimistic, we've learned a lot in the last two years.

"The virus is a problem in commercial stone fruit, but a lot of viruses have a host range, so a big concern was whether or not the virus has infected any other plant out in the woods. This is our third year of going out to the wooded areas where we know the virus was. We've sampled thousands of trees in the area and haven't found it."