Apr 7, 2007
Research Station Will Likely Lose Its Peach and Plum Fruit Trees

While no official quarantine had been announced, one was expected soon. And when it comes, Michigan State University’s (MSU) Southwest Michigan Research and Extension Center (SWMREC) near Benton Harbor, where plum pox was found in one plum tree last summer, will likely be called on to follow the Animal and Plant Health Inspection Service (APHIS) protocol and remove all susceptible prunus trees within a 500-meter radius of the infected tree.

That will take out about 12,000 trees – all the plum, peach, apricot and nectarine trees at the station, including the plums and peaches in Bill Shane’s rootstock and variety trials. Cherries are not affected by this strain (D) of the virus.

“Some plant material will be lost,” Shane said during a session on plum pox during the Great Lakes Fruit, Vegetable and Farm Market EXPO in Grand Rapids. “We’ll take our lumps.”

Shane is district Extension fruit educator at the center.

The most promising lines will be preserved, he reassured growers like the Michigan Peach Sponsors who have supported his breeding program financially and in other ways. Buds from the most promising breeding lines will be taken this winter and sent to Prosser, Wash., where they will be tested for the presence of plum pox and other viruses using the services of the NRSP-5 program. The rescue process is needed because the breeding lines are one of a kind and exist nowhere else.

“MSU will choose which to save,” Shane said. “There is a significant cost factor to consider.”

The National Research Support Project No. 5 (NRSP-5) at Washington State University tests cultivars and clones from deciduous trees from domestic and foreign sources for the presence of viruses. Virus-infected wood is cleaned up using a heat treatment process that takes time and costs from $600 to $1,300 per plant, Shane said.

Decisions at SWMREC will be made this winter. Budwood from desirable trees will be sent to Prosser and all plum pox-susceptible trees will be “destroyed before aphid time comes this next year,” Shane said. “We need to clean it up as soon as possible.”

Shane and MSU decision-makers have this winter and much of spring to make their choices.

For decades, NRSP- 5 has been the only facility performing such therapy for temperate tree fruit clones in the United States. According to its Web site, “since 1964, NRSP-5 has provided more than 1 million buds from virus-tested trees of more than 1,366 public and 663 propriety cultivars to individuals, companies and scientists in 49 states and 65 countries. Approximately 14 percent of these distributions were to commercial nurseries that increased the material in state-monitored, virus-certification programs to provide millions of virus certified trees to growers. These efforts have resulted in a dramatic reduction in virus-associated problems in commercial orchards throughout the United States.”

In addition to the breeding program, Shane has about 240 peach and 35 plum tree selections in his variety trials. The plum variety trial is duplicated elsewhere in Michigan but the peach variety trial at SWMREC is the only one in the state. Information on new varieties is usually available elsewhere, but the best information comes from trials done in Michigan, he said.

Shane said plum pox poses a special problem that makes it difficult to find solutions, other than eradication. The strain of plum pox involved, strain D, is moved primarily by aphids or by shipment of infected wood. While the rate of movement is slow and distance of movement is limited, there can be a long latent period between time of infection and when symptoms show or can be found by testing.

While in theory the trees at SWMREC could be saved by containment efforts and repeated testing, at a cost of $6 or so per sample, the huge number of buds on a tree makes it difficult to be certain that a tree is healthy. That is why rescue efforts are done using only a few thoroughly tested buds.

In all likelihood, Michigan will follow the model used in Pennsylvania after plum pox was found there in 1999, according to Mike Hansen, a Michigan Department of Agriculture (MDA) official in southwest Michigan. New York, where plum pox also was found in two orchards last summer, is in the process of making a similar decision, he said.

While the SWMREC station might be a “special case,” the case for “special treatment” was hard to make when a disease like plum pox is involved. MSU leaders will salvage what they can and go from there.

Hansen said there was no special urgency to invoke a quarantine in Michigan because plum pox was found late last season, providing lots of time before the next growing season to carefully consider state action. APHIS issued an immediate Emergency Action Notification, which set in motion an official order restricting plant movement within a 1-mile radius of the infected tree.

Since then, MDA officials have been conferring with federal officials and officials in Pennsylvania, which has been dealing with plum pox since 1999, Hansen said.

The quarantine, when it comes, will affect movement of plant material and involve testing of susceptible trees within a 7.15-mile radius of the infected plum tree at SWMREC, Hansen said. The protocol used in Pennsylvania prohibited planting of new susceptible trees in the quarantine area for three years after the last negative finding.

In Pennsylvania, continuing finds of infected trees have kept the quarantine in place, although the area under quarantine has been shrinking due to negative tests for some townships.

Both Michigan and New York are hoping for better luck, but only time and testing will tell.

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