The PPV Success Story and How to Maintain Our Quarantine-Free Status

by Dr. John Halbrendt, Penn State FREC Nematologist

Ding Dong the Witch is Dead. In the Wizard of OZ, the townspeople were jubilant when they learned the wicked witch was dead and no longer to be feared. Similarly, there was cause for celebration on October 29, 2009 when Secretary of Agriculture Russell Redding officially announced that the Plum Pox Virus (PPV) had been eradicated from Pennsylvania. This announcement came almost exactly ten years after PPV was discovered in North America.

The Plum Pox Virus (a.k.a. Sharka) is considered
one of the most economically important virus diseases of stone fruit worldwide. Many varieties of peach, plum, apricot, and nectarine produce unmarketable fruit or prematurely lose their crop when infected with PPV. Aphids are the natural vectors of the virus and it also is transmitted through infected propagation material. Commercial stone fruit is the host of primary economic importance but a number of alternate hosts have also been reported including ornamental Prunus and some herbaceous weeds and garden plants.

The first discovery of PPV in North America was from a peach orchard in
south central Pennsylvania in October of 1999. Because of the potential damage
this virus poses to the stone fruit industry, an aggressive eradication program was
implemented. The eradication program utilized intensive surveys of commercial
orchards, residential properties, woodlands, and fields to identify infected plants.
Over the past ten years more than 2 million plant samples have been tested for PPV.

Whenever a positive sample was confirmed, destruction orders were issued for the infected plant and all susceptible hosts within a 500 meter buffer zone to curtail spread of the virus. In all, some 1,675 acres of commercial stone fruit were bulldozed and burned during the eradication program. This amounted to approximately 20% of Pennsylvania’s stone fruit industry. The price tag to compensate growers for destroyed trees was over $32 million.

Infested sites were placed under a quarantine that prohibited planting stone fruit. Guidelines established by a scientific panel of experts determined that the quarantine should remain in effect until three consecutive years of negative data had been collected. This would provide a reasonable level of assurance that eradication had been achieved. In Pennsylvania, the last PPV infected trees were found in 2006 thus allowing a declaration of eradication in 2009 and removal of all quarantines on commercial and homeowner plantings.

However it should be noted that experience with PPV in Pennsylvania and elsewhere has shown that the virus is elusive and can be difficult to detect. Therefore, the guidelines also recommend that an ongoing, albeit smaller but focused, PPV survey should continue for up to ten years after the quarantine has been lifted on commercial orchards. This monitoring program is considered an essential follow-up to safeguard the Pennsylvania stone fruit industry against a second round of infection on the chance that an unknown reservoir of PPV escaped detection. As an additional layer of protection, it also is recommended that a quarantine should remain in effect for an additional three years on nursery plantings to ensure that PPV infected plants are not inadvertently distributed to growers.

Once an introduced disease has become established in a new location it is extremely difficult to eradicate. The successful eradication of PPV from Pennsylvania is hailed as a great accomplishment and could not have been achieved without the cooperation of many different groups including fruit growers, PDA, USDA/APHIS, USDA/ARS, Penn State University, and homeowners.

Lessons Learned

The appearance of Plum Pox Virus in Pennsylvania highlights the ever-present risks posed by plant virus diseases. Orchard, landscape, and nursery sanitation and good cultural practices are key issues in the prevention of virus introductions. Start with a clean planting site, purchase clean planting material and be conscientious about keeping virus reservoirs and vectors under control.

State and federal agencies have initiated clean plant programs to prevent the introduction and spread of foreign pathogens. Best management practices require growers to be familiar with common disease problems and their control. It also is important to remain vigilant for anything unusual and bring it to the attention of experts who can identify the problem.

Penn State has a web site that provides cultural and pest control information for Prunus ornamental and fruit producers. Visit the plum pox site to read about:

• Lessons Learned from Plum Pox Virus—Preventing the Introduction of Exotic Plant Pests
  
• Re-Inventing Our Peach Industry—A Chance to do Everything Right
  
• Replanting Prunus in Sites Previously Affected by Plum Pox
• Introducing New Plants—Not Pests—to Your Orchard

You may also contact your Penn State Cooperative Extension office for copies of brochures on these topics.

Maintain Control of Virus Vectors by Regular Sampling for Nematodes
A new nematode assay form is available in the 2010-2011 Pennsylvania Tree Fruit Production Guide. Please note that the fee for processing a nematode assay has increased to $20 per sample beginning January 1, 2010. A copy of the new form can also be obtained from your county extension office or the Fruit Research and Extension Center in Biglerville.