Plum Pox Virus in Michigan
By Mark Longstroth
As of September 19, 2006 no new plum pox infected trees have been found in Michigan.
A special Plum Pox edition of the Fruit CAT Alert was published in August 2006 and is also available off the MSU Fruit IPM Resource Page.
Plum pox, a stone fruit virus that is harmless to humans, was confirmed in Michigan in August of 2006. Plum pox was first found in North America in Pennsylvania in 1999. The Michigan Department of Agriculture (MDA) has participated in a national surveillance program for plum pox virus (PPV) since 2000. The discovery of a single infected plum tree at the Southwest Michigan Research & Extension Station (SWMREC) was the result of this ongoing survey effort. At this time the MDA is sampling all host plants in the area to determine extent of the disease. The infected tree was part of a plum variety trial at SWMREC. Trees in the same trial in Michigan planted in another location were also sampled and no plum pox was found in the second planting. The current tests used to discover this disease are accurate and fast. Within a few weeks, we will know if plum pox is restricted to one tree or is more wide spread. The MDA hopes to sample all possible hosts within five miles of the infected tree by the end of the 2006 growing season, to determine if it was an isolated tree that was infected when it was planted or if it was infected after planting from undiscovered infected trees near by. Once the extent of the disease is known, control measures will be determined. Normally this means the destruction of infected and nearby trees that can harbor the disease. The USDA's Animal Plant Health Inspection Service (APHIS) is also involved in the detection effort and determining the spread of plum pox in the United States.
Plum pox is a significant threat to the stone fruit industry in the United States. Peach and plum trees are most at risk from the strain of the plum pox found in North America. This virus is considered the most serious virus disease of peaches and plums in Europe. Plum pox affects the growth of infected trees. Leaves and fruit often show chlorotic (yellow) rings. Affected fruit is often unsalable. There is a wide range of symptoms and some plants may have the disease and show no symptoms. The infected plant in Michigan did not show any symptoms. Infected plants are typically destroyed to prevent the spread of the disease. Long distance spread of the disease is by infected plants carried by people to new areas. Once an infected plant is planted in the area the disease can be spread by some species of aphids.
Plum pox was first found in North America in Pennsylvania in September of 1999. The USDA launched an eradication effort there searching for infected trees and destroying both infected trees and near by susceptible hosts. Canada at first embargoed nursery trees from the US but soon found plum pox in Ontario in 2000. The disease was first described in Bulgaria in Southern Europe in 1915. It spread slowly until after World War II. By the mid 1980's it had spread to most of Europe. In 1992, it was found in Chile in the Western Hemisphere. The USDA has pursued a vigorous policy of eradication in the US. Plum pox was recently found in New York in one plum tree growing near Lake Erie and the Canadian border.
MDA has received GREEEN Grants to survey the state for PPV and establish that Michigan is free of the disease. This does not mean that every peach, nectarine, and plum tree in Michigan is tested. The MDA has focused their tests on trees that are used for budwood for new trees and on orchard trees near nurseries. MDA also surveys known stone fruit orchards in the state. It was this survey of stone fruit plantings that discovered the infected tree at SWMREC. In 2000, MSU developed a pocket detection guide for growers (Extension Bulletin E-2718) so they can recognize the disease and report it. Visual monitoring of the orchard by growers is an important part of any disease control program. Growers see their trees more than anyone else does.
Plum pox does not pose a threat to humans or any animals. Plum pox is a virus that infects only plants. The "D" strain of plum pox is found in North American is one that is less virulent and does not affect cherries. The virus is spread from tree to tree by green peach aphids and only a few other aphids speicies. Most insects cannot spread this disease. Aphids can only spread the disease a short ways, only a few hundred yards. Long range spread from state to state or country to country or across the oceans is by the spread of infected plants or cuttings by humans. Not all stone fruit show distinct and dramatic symptoms so spread across the world has prbably been by people carring plants they want to grow that are symptomless carriers of the disease. The MDA, MSU and MSU Extension are working right now to determine what is the extent of the diseases trees need sampled and the number of samples needed.
This virus is a serious threat to the peach and plum plantings in Michigan. The Michigan Peach Industry is worth about ten million dollars in income to growers. Production is centered in Berrien and Oceana counties with almost 2000 acres in each county. In these counties peach production is an import part of the local economy. In Berrien County most of the fruit is sold as fresh fruit in Michigan and across the Midwest. Many Oceana County peaches are processed into frozen peach slices. Across Michigan, local peaches are sold locally at farm markets. There are over 5,000 acres throughout Michigan on 470 fruit farms. Peaches represent an important crop on these farms, increasing the diversity of fruit crops planted, reducing the risk of total crop loss and spreading income over several sources. The Michigan Plum Industry has declined about 50% in the last several decades with the decline of demand for both fresh and processed plums and prunes. But plums are still a significant crop for the growers who have them. Plums generate about a million dollars of income each year. 180 fruit growers farm about 900 acres of plums.
A special Plum Pox edition of the Fruit CAT Alert was published in August 2006.
For more information on Plum Pox Virus see the APHIS Plum Pox Site.
Also see the Plum Pox Resources Page at the University of West Virginia's Kearneysville website.
