Apr 7, 2007

Virus Found in New York, Michigan

After causing a stir six years ago, plum pox virus (PPV) laid low while government officials conducted mopping up operations in Pennsylvania and Ontario.

Then, within a month this summer, PPV was found in New York and Michigan, propelling a whole new outburst of concern.

PPV is a serious plant disease affecting stone fruits, including plums, peaches, nectarines and apricots, but the strain involved in the North American outbreaks, strain D, does not affect cherries. The virus disfigures fruit, reduces yield and greatly shortens tree life, but poses no threat to human or animal health. The disease infects an estimated 100 million trees in Europe and is a major barrier to stone fruit production there. The United States is trying diligently – by testing, quarantine and eradication – to keep this disease (sometimes called by its Slavic name Sharka) out of the country.

The USDA’s National Plant Germplasm and Biotechnology Laboratory in Beltsville, Md., using ELISA testing, on July 17 confirmed the presence of the PPV on two plum tree samples in New York’s Niagara County. On Aug. 11, it did the same for a sample from Berrien County, Michigan. On Aug. 21, a sample from a peach tree 11 miles east of the first New York finding was also confirmed positive for PPV.

Plum pox is a viral disease of stone fruit species that was first found in Macedonia in 1910 and has since spread to other countries, hitting hard the traditional plum-growing areas of southeastern Europe as it moved west into Spain and France, east to India and south into the Middle East.

It first appeared in the United States in Pennsylvania in October 1999 and then in Ontario in 2000. The New York and Pennsylvania discoveries were made in commercial orchards and the Ontario find was in a nursery.

The Pennsylvania outbreak, the first and largest, has cost an estimated $40 million, some $26 million in indemnities to growers who have lost 1,599 acres of fruit, according to official summary figures supplied by Nancy Richwine in the Pennsylvania Department of Agriculture.

Ralph Scorza, a USDA horticultural researcher at the Appalachian Research Station in Kearneysville, W. Va., said it’s difficult to tell what the discoveries mean. It could be the tip of the iceberg, he said, as it was in Pennsylvania, or just isolated events.

The Michigan plum tree sample was collected at the Southwest Michigan Research and Experiment Center (SWMREC), a Michigan State University facility located near Benton Harbor. The finding threatens SWMREC’s future as a peach breeding and testing station and the fate of 14,000 stone fruit trees there.

As was the case in New York, the Michigan samples were collected by state agriculture department officials as part of routine state surveillance surveys conducted in cooperation with USDA APHIS (Animal and Plant Health Inspection Service). The surveys have been on-going since the Pennsylvania discovery.

The plum pox strain identified in all locations is the D strain – less virulent than other strains, not infecting cherry trees and not seedborne. Because the strain is not seedborne, it is not necessary to regulate the movement of fruit to prevent the spread of the disease. Growers may continue to harvest and market fruit.

The New York and Michigan findings set a whole clockwork of gears into motion.

Survey specialists immediately began surveying all 14,000 potential host trees at the SWMREC facility. Following the completion of this survey, APHIS and Michigan Department of Agriculture (MDA) planned to expand surveillance efforts to include host trees within a one-mile radius of the center. Already by Aug. 20, federal Emergency Action Notices were being served to all orchardists and homeowners within the mile restricting the movement of certain species of Prunus fruit wood.

A meeting was held Aug. 16 with local growers at the SWMREC facility to explain what would be happening at their farms.

A similar process took place a month earlier in New York.

There, as part of a seven-year survey for the virus, state and federal agriculture officials collected 22 leaf samples from a 108-tree orchard in Niagara County, N.Y. – within five miles of plum pox eradication zones in Canada. The samples were sent to Cornell University’s diagnostic laboratory for testing, where researchers obtained positive results later confirmed by the USDA lab.

Survey specialists began surveying a five-mile radius surrounding the initial detection to determine the extent of infestation. In an expanded survey covering 25 percent of the trees, the infected peach tree was found 11 miles away.

The program will include conducting extensive detection and delimiting surveys, establishing quarantine areas where infestations are found and removing infested orchards and other host material (wild and ornamental plantings) within a buffer area of any infestation.

The two infested trees in New York, both Castleton plums, have been destroyed, but no such action has yet been been taken in Michigan. Officials said that, given the lateness of the season, the slow-moving nature of the disease and the excellent spray and management program at the research station, the infected tree poses limited danger.

Priority should go to testing to find any other infected trees, said Mike Hansen, Michigan Department of Agriculture regional supervisor, and David McKay, USDA-APHIS state plant health director for Michigan, since there are only about 30 days left before frost and leaf fall.

The finding came late in the season, Hansen said. The last samples were taken July 24 – and one of those “came up yellow,” he said.

Bill Shane, the Michigan State University district fruit and marketing agent at SWMREC, said that while PPV is not a familiar disease in the United States, a lot is known about it from its habits elsewhere.

The PPV virus is spread by aphids and through infected wood. Several aphid species can carry the virus, the spirea aphid being one of major concern in Pennsylvania. Aphids probe and test plant tissue as they look for plants they like to feed on. The plum pox virus, picked up from an infected tree, stays viable in an aphid’s mouthparts for about an hour. Transmission is usually limited to about 30 to 160 feet from the initial source plant, Shane said. USDA uses 500 meters, about three-tenths of a mile, as a zone of eradication surrounding an infected tree.

Deborah Breth, an Extension educator in the Lake Ontario Fruit Program, said that the New York State Department of Agriculture and Markets and USDA worked closely to determine the extent of infection in the Niagara County area, in the town of Porter. In late July, only two trees in the 108-tree plum block were identified as infected.

The initial trees were treated with aphicide, then destroyed and the trunks treated to prevent sprouting. Within a 2.25-mile radius of that orchard, every Prunus tree except cherries will have a leaf sample collected (eight leaves per tree). Between the 2.25- and 5-mile radis, four leaves per tree from every Prunus tree will be collected (two trees per test) and tested for PPV.

A similar protocol will be used in Michigan, starting at the SWMREC station and moving outward.

This is a slow spreading disease when transmitted by aphids, but can be spread in budwood and transferred horticulturally, Shane and Breth said. Aphids are mainly responsible for the short-distance transmission of the virus from infected areas to uninfected areas.

Humans have been responsible for some of the greater-distance spread where the disease has crossed natural barriers like mountains and oceans. Mira Danilovich, the MSU district horticulturist in west central Michigan, said the initial infections in the U.S. “probably arrived in someone’s suitcase” as someone of European background brought wood from a favorite variety from his or her homeland.

If budwood collected from an infected area is used for grafting and budding new trees, virus-infected budwood will result in a PPV-infected tree being transported to various locations. It is critical that any budwood collected or shared for the purpose of grafting new trees is certified clean of the PPV, both Shane and Breth said.

Don Albright, the APHIS national operations director for PPV located in Carlisle, Penn., said that after the Pennsylvania outbreak APHIS conducted a three-year nationwide survey for PPV and found nothing. Surveys have continued in states with high fruit production or a sizeable nursery business.

Typically, he said, 10,000 to 25,000 samples are taken in a state. California, New York, Maryland and Michigan were surveyed this year – with the resulting two finds. In Pennsylvania, 250,000 samples were taken last year and five infected trees were found in already-quarantined areas. That brought to 454 the total number of infected trees found in Pennsylvania since 1999.

Whenever infected trees are found, tracing work begins backward, to find where the tree came from, and forward, to find where budwood might have gone.

When an infected tree is found, all trees in the block are destroyed and so are all trees within 500 meters of the block.

Infected trees take some time to show the virus. A tree infected by aphids may show symptoms on a leaf or limb the first years and it may be three years before the entire tree is infected. Sampling, which takes a few leaves from a tree, could miss finding an infected tree.

Albright was on his way to Michigan Aug. 15 after having just returned from New York, where he and about 10 others collected 30,000 samples in four weeks. Eight leaves are usually taken per tree, two off each quadrant in a typical four-scaffold tree.

“We have to figure out how the virus got into Michigan,” he said. “Up until a month ago, the only place that had it was Pennsylvania.”

In the Michigan research orchard, good records kept by researcher Bill Shane may help in the traceback. The plum tree was planted in 2003.

But, Shane said, it cannot be assumed this is the source tree. Another tree, not yet found, could be the source and this tree infected by aphids. While he knows the variety of the tree and its source, it’s not yet time, he said, for a witch hunt looking for a nursery spreading a disease.

“It could have come here infected, or it could have been infected here,” he said.

Robin Rosenbaum, the plant industry division manager in the Michigan Department of Agriculture, is the “incident commander” coordinating the Michigan investigation. She described the process – the cooperation among agencies, the tracing backward and forward, the sampling, the looking for connections.

The Canadians use a similar process. After months of exhaustive research, the source of the PPV outbreak in Ontario was traced to a nursery in Niagara-on-the-Lake, according to officials from the Canadian Food Inspection Agency.

Budwood from two clingstone peach varieties in the nursery was used for propagation of trees that were subsequently distributed across Ontario. Most of the 12,000 to 13,000 affected trees, including 8,000 from one grower in Niagara-on-the-Lake, were removed in 2001 to stem the outbreak.

Most of the affected trees were from Niagara, but some trees were found near Port Dover and Chatham in Ontario and four in Nova Scotia. Approximately 80 growers were forced to remove and destroy their trees.

Shane said that grower vigilance can sometimes be more effective at finding diseases like PPV than testing. That are dozens of pictures on the Internet showing symptoms of the disease on leaves and fruit of peaches, plums and apricots. The PPV symptoms look somewhat like powdery mildew and San Jose scale.

Severely infected trees have smaller, oddly marked fruit with less sugar that may taste bitter and have less flavor. Severe fruit drop greatly reduces yield, he said.