Generally, it is very difficult to isolate and control such global diseases, let alone eradicate them completely from an area. However, through an impressive cooperative effort involving dedication and scientific innovation, the Plum Pox Virus was eradicated from Pennsylvania in 2009.
"Pennsylvania is unique, perhaps, because it was the first detection sight in North America, and the only substantial infestation in the world to reach the eradication threshold up to this point," said Ruth Welliver, Plant Pathology Program Manager at the Pennsylvania Department of Agriculture.
In 1918, the most destructive disease of Prunus fruit and ornamental trees first emerged in Bulgaria, called the Plum Pox Virus (PPV), or Sharka. When it first hit, peach experts from around the world predicted that it would be impossible to eradicate. They knew very little about the mechanism of transmission, and what to do to prevent its spread.
In the decades following its emergence, it quickly became apparent that the virus could devastate large numbers of stone fruit trees in a short period of time. It quickly spread to various other regions in Europe; it hit France and Italy in the 1970s and Spain and parts of the Middle East in the 80s. In Europe, Plum Pox Virus has destroyed more than 100 million stone fruit trees, ravaging orchard yields and heavily diminishing the stone fruit industry.
In 1999, Plum Pox finally reached North America, manifesting in a peach orchard located in Adams County, Pennsylvania. "I first spotted a strange ring pattern on some of my peaches and wasn't sure what could have caused it," said said Jim Lerew, co-owner of the orchard with the first known case of Plum Pox infection. He continued, "After I found it several more times on my peach crop, I took some samples to a number of different fruit research institutions, including Penn State, to determine the cause, which admittedly stumped them for a while. No one had even considered that it could be a disease that originated in Europe." Infected orchard areas were contained and quarantined and infected trees were burned to inhibit further disease spread.
"Subsequently, the virus was also found in Ontario and Nova Scotia, Canada, and then in New York and Michigan. In Michigan and Nova Scotia, a very small amount of the virus was found and it was eliminated quickly. Pennsylvania had a larger infestation that took longer to eradicate," recalled Ruth Welliver.
Plum Pox Virus does not kill infected trees outright; it causes severe yield losses to growers and greatly reduces the marketability of stone fruit. It is spread over short distances by aphids and spread over long distances via infected nursery stock or through infected buds grafted onto healthy trees.
For successful short distance transmission, an aphid must first feed on an infected plant, acquiring sufficient amounts of the virus in its system through its mouthpart, called the stylet. The stylet probes into the vascular tissue of the plant during feeding and spreads the disease through nonpersistant transmission. Nonpersistant transmission refers to the fact that the virus is extraordinarily infectious, but only for a short interval and it can only be transmitted in the time it remains infectious. The lifespan of the virus within an aphid is less than one hour; thus the aphid must fly immediately to a new host plant and inject its stylet into healthy plant cells for successful transmission. When this is complete, the aphid no longer carries the virus, until it feeds on another infected plant.
Stone fruit species--or members of the genus Prunus--that are susceptible are: almonds, apricots, cherries, nectarines, and plums, along with some ornamental plants and wild species. Stone fruit are most susceptible to strain D of the virus in North America, with the exception of cherries (they are not affected by strain D at all), although there are several other strains that are more aggressive and destructive that can be found in other areas of the world.
A group of fruit growers, state and federal agriculture departments, and members of the Penn State College of Agriculture came together in a cooperative effort for the difficult task of eradicating sharka. "The Pennsylvania Department of Agriculture, U.S. Department of Agriculture, and Penn State University provided a sound scientific base and also served in regulatory and educational roles. A host of other contributors from the local, state, and federal levels worked in the political arena to ensure that there was political support and monetary funding," said Ruth Welliver. This dedicated coalition recognized that the establishment and unmitigated spread of Plum Pox Virus in the United States would jeopardize the nation's stone fruit industry, as well as diminish commercial nursery production and the use of ornamental Prunus in home landscapes.
Fall 2009 marked the third consecutive year of no new positive detections of Plum Pox, marking the end of the three-year long quarantine. Pennsylvania was thus dubbed Plum Pox-free. "A major milestone has been reached for Pennsylvania agriculture and this historic moment is a testament to teamwork, perseverance, and science…Because of the efforts [of many] we will continue to enjoy peaches and other stone fruit from Adams, Cumberland, Franklin, and York counties," said Secretary of Agriculture Russell Redding. Bruce McPheron, Dean of Penn State's College of Agricultural Sciences, added that "the Plum Pox eradication effort is a perfect illustration of how the state's land-grant university can mobilize research and generate practical knowledge for the public good…this successful collaboration can serve as a blueprint for future plant-and animal-disease outbreaks."
"The Plum Pox epidemic served to raise the level of awareness that all growers need to possess," said John Halbrendt, Penn State Fruit Research and Extension Center scientist, "orchard sanitation and good cultural practices are key issues in the prevention of virus introductions."
The Plum Pox Eradication Project Team provided a service to the nation's stone fruit industry by proactively addressing and preventing loss and damage from pests and disease outbreaks on a large scale. The eradication of Plum Pox in 2009 was a notable scientific effort that resulted from remarkable cooperation on the part of the project team.
On August 3, 2010, U. S. Agriculture Secretary Tom Vilsack presented the esteemed Honor Awards for a number of leadership efforts and accomplishments that have furthered the mission and goals of the Department of Agriculture. He explained, "The Honor Awards are the most prestigious awards given in the Department. They are given to those who represent outstanding service in many fields." One such award was presented to the locally based Pennsylvania Plum Pox Eradication Project Team. This award recognized not only U.S. Department of Agriculture scientists but also their grower, Pennsylvania Department of Agriculture, and Penn State Extension partners.