Aug 30, 2007
Battle to eradicate plum pox continues in three states, Canada

The battle to rid North America of the plum pox virus, first detected in Pennsylvania in 1999, is proceeding unabated in the four areas where it has been found. The burden imposed on growers by quarantines has been reduced and life is returning to normal for them.

Two different approaches are being used because two national governments are involved. The Canadians are using slightly different methods to eradicate plum pox from southern Ontario than the Americans are in Pennsylvania and Michigan and in nearby Niagara County in New York. While there is some “suspicion” among U.S. growers about Canadian methods, government officials on both sides praise the “other” approach.

The discoveries of plum pox in Michigan and New York last year were isolated finds that have not risen to the epidemic levels that occurred in Pennsylvania and Ontario.

Don Albright, national coordinator of federal eradication efforts within USDA’s Animal and Plant Health Inspection Service (APHIS), said that sampling continued this year in Michigan, Pennsylvania and New York – without finding any more infected trees.

In New York, one sample is being re-tested. Even if confirmed positive, it would bring the total number of infected trees found in the state to four, Albright said.

In Michigan, one infected tree was found in July 2006, but it had a profound impact because the finding occurred at Michigan State University’s Southwest Michigan Research and Extension Center in researcher Bill Shane’s peach breeding and variety evaluation trials.

Mike Hansen, regional supervisor for the Michigan Department of Agriculture, said that all the prunus trees at the station have been removed except for “a few trees under a screen.” To save the most promising lines in Shane’s peach trials, budwood is being collected and sent to Prosser, Wash., where virus-infected wood can be cleaned up.

Wood from several trees has already been sent to Prosser, Hansen said, but one or two lines remain to be salvaged. These have been covered with screens to prevent access by aphids, the primary carriers of plum pox virus from tree to tree.

In Michigan, as in other states, the government has taken over the major role in finding and quarantining infected trees in commercial orchards, Albright said. The federal government has taken on the difficult role of finding infected trees in residential settings, abandoned orchards and in the wild.

Besides commercial peaches, plums, apricots and nectarines, this strain of plum pox also infects wild species and those used in landscaping.

Hansen said samples were taken from 50,000 trees in commercial orchards in southwest Michigan and 30,000 trees in west central Michigan – all without finding a single infected tree.

A 5-mile diameter circle around the research station is under quarantine, Hansen said, meaning that no prunus wood can move out of the area without a permit – and no new trees can be planted in that area without a permit. But growers can get free permits to plant.

“We want to track what’s coming in,” Hansen said, “but growers can plant new orchards in the area.”

In Pennsylvania, Albright said, portions of three townships in Adams, Cumberland and York counties remain under quarantine, and growers can’t plant or replant prunus orchards (except cherries, which are not affected by this Strain D) in that area. At the peak in 2003-04, he said, 352 square miles of land were under the replant ban. That has been reduced to 94 square miles. If no new positives are found this year, it will be reduced further to three small areas totaling 40 square miles.

About 400 infected trees were found in Pennsylvania in the fall of 1999, a further indicator of the relative size of the outbreaks. No one really knows how many trees were infected, Albright said, because the “cookie cutter approach” was used in eradication.

Finding an infected tree resulted in the removal of all susceptible prunus trees within 500 meters – nearly a mile-wide circle – and a ban on replanting in the quarantined area.

The Canadian approach has been different. Thierry Poire, an agriculture specialist with the Canadian Food Inspection Agency in Ottawa, said the U.S. approach would have killed the Canadian industry. About 80 percent of the peaches, plums, apricots and nectarines in Canada are grown by about 600 growers clustered on the peninsula between Lake Erie and Lake Ontario.

When the plum pox discovery was made in 2000, just after the Pennsylvania discovery, the Canadians launched a program to first determine whether eradication was even possible, Poire said. While Pennsylvania was testing 250,000 samples that year, Ontario tested 1 million, Albright said.

While no exact number is known, Poire said that “thousands of positive trees” were found, more than in Pennsylvania.

The program the Canadians settled on was at first called “suppression” rather than eradication. All infected trees were removed. Later, the program escalated to eradication mode, in which all infected trees were removed – as well as all trees in the same block. It was suspected that the virus originated in infected budwood, so removing an entire block of related wood was a logical approach.

“It is going well,” Poire said. “The incidence of disease is going down; numbers are going down.”

Still, “only” 100 positive trees were found this year in the Canadian testing program, Poire said.

Canada is working hard to make sure the prunus industry in the area survives, he said. There is a good indemnity program, which compensates growers for lost trees, the cost of new trees and replanting and for income lost during the restoration period.

When infected trees are found, they are removed immediately, but the grower does not have to remove the block until after harvest.

In addition, replanting is allowed the year following removal – except in the case of one variety of nectarine that suckers a lot, Poire said.

“Canada has been working with us right along since 2000,” Albright said. “We trade ideas and there has been a lot of cooperation. But our approach is different.”

There would be some benefit if we could “harmonize the region,” he said. New York growers are concerned that the Canadians use smaller buffer areas and replant more quickly. Albright said the Canadian approach may take longer and is more expensive, but understands the Canadian concerns. A more aggressive approach could, he agrees, kill the Canadian peach industry. The Canadians seem willing to invest the time and money needed to prevent that from happening.

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