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Plum pox virus, a stone fruit disease first found in the United States eight years ago, has the potential to spread to major fruit-growing regions such as Washington State unless more safeguards are in place, warns Phil Baugher, vice president for marketing at Adams County Nursery in Aspers, Pennsylvania.

The virus affects peach, nectarine, plum, apricot, and cherry as well as ornamental Prunus species. In most varieties, it disfigures the fruit, making it unmarketable. It also reduces productivity and causes premature drop.

It is primarily spread by aphids, which carry virus particles in their mouth parts while feeding, but can be spread over greater distances in infected grafting wood or nursery stock.

The first known infection in the United States was in an Encore peach orchard in Adams County, Pennsylvania, in 1999. Within two years, more than 1,000 acres of stone fruit orchard had been removed in an effort to eradicate the virus, and another 500 acres have been removed since then. The first growers to remove their orchards in the winter of 1999-2000 will be allowed to begin replanting some of their acreage this year.

A separate outbreak of the virus was found in 2000 in Ontario, Canada. In 2006, a random survey uncovered a single-tree infection at the Southwest Michigan Research and Extension Center and three infected trees at two locations in western New York.

Despite trace-back efforts, officials have failed to determine the entry pathway for any of the infections, Baugher said.

Europe

The virus was first reported in Bulgaria in about 1915. By 1960, it had spread throughout eastern Europe and into the Netherlands, Switzerland, England, Greece, and Turkey. A decade later, it was in France, Italy, and Belgium, and by the 1980s, it was in Portugal and Spain, where it was thought to have entered on imported plum rootstock. Though the virus mainly affected plums and apricots at first, it was later observed in peaches. It turned up in Chile in 1992 and India in 1994.

Baugher said that in Spain, a lack of industry resolve resulted in widespread infection and no possibility of containment or control. In France, the government inspects orchards, and infected trees are immediately destroyed. Although this has lowered overall infection, it is costly and represents a significant portion of growers’ overall production costs. The strain in France is PPV-M, an aggressive strain not yet seen in North America. The D strain, found in Pennsylvania, was thought to thrive only in peach, nectarine, plum, and apricot, but laboratory studies have shown that it can infect cherry.

Baugher thinks there’s reason to be concerned that if the virus were introduced into the Pacific Northwest, it might change over time and infect cherries, which are the major stone fruit crop in the region.

Clean material

In Europe, the first line of defense is clean plant material, he said. In France, for example, all bud wood must originate from registered trees, and every tree must have trace-back information to the mother tree when delivered to the customer. All new plantings must be from certified virus-free sources.

Baugher said certification in the United States is piecemeal at best, as there are no federal certification standards for tree fruit nursery production. Regulations vary from state to state. Washington has a recognized and very effective program, he said, but to some states, “certified” might mean simply that a state official has visited the facility and inspected the plant material for the visible presence of insects or diseases.

The presence of plum pox virus in North America has raised questions about the need for more safeguards. There are concerns about virulent strains such as PPV-M arriving in North America in the same way as the other strains.

Though importation restrictions are helpful, plant material still comes into the country undetected or undeclared, Baugher pointed out.

“In an industry with ever-increasing grower participation in global partnerships for variety introductions, there is an increased risk of quarantine pests crossing our borders,” he said.

Funding

The tree fruit industry has been well served through Washington State University’s National Research Support Project No. 5 at Prosser, Washington, and the Foundation Plant Services at the University of California, Davis. These programs provide virus testing and treatment services for material from domestic and foreign sources, Baugher said, but, he added, maintaining adequate funding is a struggle.

The National Plant Board, which is made up of the principal plant pest regulatory officials of each state, has recommended that the U.S. Department of Agriculture establish a national standard for nursery certification.

Legislation before Congress would establish a National Clean Plant Network with funding from the Cooperative State Research, Education and Extension Service. Under the new network, NRSP5 and FPS would be the primary quarantine sites providing virus elimination and testing services. Blocks of elite plant material would be established around the country to serve regional needs.

Continuation of the NRSP5 program depends on Congress funding the new network, as federal funding for NRSP5 is being reduced each year, and is scheduled to end on September 30, 2008, said NRSP5 manager, Bill Howell.

“We’re hanging from a cliff. We haven’t closed the doors, but it’s kind of a stressful time for the personnel here.”

Don Albrecht with USDA Animal and Plant Health Inspection Service has estimated that $40 million were spent during the first three years of trying to rid the country of plum pox virus, and that clean plant programs potentially contribute more than $27 million annually in the form of reduced grower losses and increased availability of fresh fruit to the consumer.

Impact on nurseries

Howell said the introduction of plum pox virus to the Pacific Northwest would not only be devastating to orchardists, but could have serious political and trade implications for the nursery industry, which might be prevented from shipping bud wood or trees from infected areas, as Adams County Nursery has been. Washington is home to many of the largest tree fruit nurseries in the country. A loss of orchards and nursery business would in turn impact the businesses that provide services, such as chemicals and equipment, and the work force. “It’s really hard on communities,” he said.