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The lone discovery of plum pox virus in a peach tree in Michigan State University’s tree fruit breeding program puts significant constraints on the program, but it’s not all "gloom and doom," says Bill Shane.
The virus, found last July at the Southwest Michigan Research and Extension Center in Benton Harbor, has resulted in the testing of more than 12,000 susceptible trees at the station and will require that most of the trees be removed. As of press time for Good Fruit Grower, quarantine regulations were not yet issued, although state officials expect that movement of Prunus material within about a seven-mile radius of the positive find would be prohibited. The quarantine is not expected to be lifted until all trees test negative for three subsequent years.

"We will lose a lot of trees at the research station," Shane said, but added that they are working to save the more unique and most valuable advanced selections from MSU’s breeding program. As district Extension fruit and marketing educator, Shane is responsible for tree fruit research at the center, including the peach and plum breeding programs.

MSU officials plan to identify the top Prunus breeding lines—the most advanced and promising peach and plum selections—and send wood to the National Research Support Program No. 5 in Prosser, Washington. The Prosser facility provides virus testing and certification of deciduous tree fruit and is the main source of virus-free bud wood in the United States.

A Michigan Department of Agriculture tree survey, which sampled every susceptible tree at the Southwest Research Center for the virus, detected only one infected tree, but one was enough so that the research material must be eliminated, except that which will be sent to Prosser for testing. The plum pox virus found was strain D, which does not affect cherries or tree fruit other than peaches, plums, apricots, and nectarines.

"As a result of the virus, we’ll take out most of the trees on the station," Shane said, adding that projects there include plum and peach rootstocks and peach, plum, and apricot variety trials. "The peach and plum variety trials will be removed because we can’t take a chance that the virus has spread to other trees. We have to be thorough because it can take up to three years or more for trees to show symptoms due to the virus’s slow spread and long latent period."

Other locations

But he noted that the MSU peach breeding program does have some of their selections safely at other locations, and additional plum research work is under way in west central and northwest Michigan.

"A bright side is that as a result of the virus, we will move some of the advanced peach breeding selections forward faster to release as new varieties because all new varieties must be tested for pathogens before nurseries will handle them," he said.

Shane estimated it would cost between $600 and $1,300 per plant for the Prosser testing. Because of the expense, they can’t test every selection, and instead, will have to choose which selections to keep. He expects that some of the most advanced peach selections will be chosen for further testing at Prosser.

By last September, all samples from the research center had been tested. The one original plum tree was still the only positive sample, reported the Michigan Department of Agriculture. The positive tree, which was caged with a fine screen to prevent aphids from further infecting other trees, was sprayed with herbicide to kill the roots and then destroyed.

Since the positive find in Michigan, some 50,000 samples have been taken, representing 100,000 trees, according to agriculture department officials. Testing focused on bud wood used for new trees and orchards near nurseries. State officials have been testing susceptible trees in Michigan since 2000.

Michigan’s peach industry, with more than 5,000 acres, generates about $10 million in income to growers, according to the state agricultural department. Most of the fruit is sold as fresh product in retail and farmers’ markets in Michigan and across the Midwest. The state’s plum industry is much smaller, generating about $1 million of income from an estimated 900 acres.