Plum pox virus (PPV) was discovered for the first time in North America in 1999 in a peach orchard in Adams County, Pennsylvania. In Pennsylvania, peach, plum, nectarine, and apricot stone fruit and native and ornamental Prunus are susceptible to PPV, Strain D.
A Devastating and Economically Important Disease of Stone Fruit
Plum Pox or "Sharka" is one of the most devastating and economically important diseases of stone fruit worldwide. The disease is caused by the Plum Pox Virus (PPV) which is spread by aphids and by the use of infected propagation material.
Results of surveys indicated that within the United States the virus was initially confined to a relatively small geographic area of Pennsylvania, although later the virus was discovered in New York. In Pennsylvania, mandatory destruction orders were given for all PPV infected orchards as well as for stone fruit grown within a 500 meter buffer zone of infected sites. The virus affected regions were placed under a quarantine that prohibited new plantings of susceptible stone fruit.
Considerations for Replanting Prunus
A Chance to Do Everything Right!
All new orchards offer the opportunity for a "fresh start" and growers must plan carefully to get the most from their investment. Key points to consider include:
- Evaluate the location for any site related problems such as air drainage, slope orientation or soil moisture issues.
- Evaluate the replant site for potential nematode / soil-borne disease problems.
- Evaluate the replant site for potential pH / fertility problems.
- Review the site cropping history for indicators of previous production problems.
Identify Potential Soil-Borne Problems
A soil test and a nematode assay are essential first steps in identifying potential soil-borne problems. The nematode assay will determine if damaging levels of plant-parasitic nematodes are present. If nematodes are a problem the grower needs to choose a management option that fits into the renovation plan.
Nematode control options are typically soil fumigation, nematicides or some combination of rotation / green manure crop. The choice of nematode control will be influenced by the specific nematodes that need to be managed, the future crop to be planted, the time frame for site renovation, efficacy and cost.
A soil test report will provide recommendations to correct any nutrient imbalances or pH problems. These corrective measures need to be started before the new orchard is planted so that fertilizers and lime can be incorporated into the soil for maximum benefit. This is particularly important for P, K, Mg and Ca fertilizers as well as Zn, Cu, Mn, Fe and lime that move very slowly down the soil profile.
Prepare a Replant Time Table
Once potential replant problems have been identified and corrective measures decided upon, it is beneficial to prepare a time table that includes each major step in the renovation process. Among other things, the time table helps to ensure that an adequate amount of time is allocated to accomplish each task.
Special Considerations for Replanting Prunus with Regard to PPV and other Virus and Virus-like Diseases
Plum Pox and other plant viruses are difficult or impossible to control once they are in a field setting since the only way to eliminate the virus is to destroy the host. Therefore, disease prevention is the only practical management strategy to avoid virus problems.
Endemic virus diseases such as Tomato Ring Spot Virus (ToRSV) are widespread and have many alternate hosts that can serve as reservoirs. It would be impossible to eradicate ToRSV from the landscape and therefore prophylactic measures to prevent virus spread are the only option for growers. However the PPV that was introduced into Pennsylvania offered a unique opportunity for eradication because of its limited distribution and limited host range.
Eradication can only be accomplished if all PPV reservoirs are identified and destroyed. The restriction on planting Prunus within a quarantine zone limited virus access to susceptible hosts and is an invaluable tool in the eradication process. The decision to rescind the quarantine and allow replanting was made based on the results of several years of intensive surveys that indicated the area was free of PPV. Thus replanted orchards on previously quarantined sites should be at no greater risk of contracting PPV than sites that had never been under quarantine. Nevertheless, the possibility that some reservoir plant escaped detection can not be completely dismissed and PPV surveys will continue even though the land is no longer under quarantine.
Purchase Certified Virus-Free Trees
Whether the goal is to prevent PPV or another common virus such as ToRSV, there are several precautions that growers can take to avoid infection and prevent spread. Perhaps the single most important consideration is to purchase certified virus-free trees from a reputable nursery. It is important to inquire which viruses are included in the virus certification since each virus requires its own test.
Any virus that is not included can escape detection. Plum Pox and common viruses such as Prune Dwarf (PDV), and Prunus Necrotic Ring Spot (PNRSV and ToRSV) can easily be spread in the nursery by propagating infected tissue. Infected nursery plants have been responsible for the introduction of virus diseases into new countries and regions.
Control Potential Reservoir Hosts
Another virus management consideration is the control of potential reservoir hosts. The natural host range and the role of weeds in PPV survival and spread are not fully known especially in locations where the virus was introduced such as Pennsylvania. However in Europe wild Prunus can function as a symptomless PPV reservoir making eradication of the virus impossible in many areas.
There have also been reports of PPV infections in herbaceous plants. Although no PPV infected weeds have been detected in the field in Pennsylvania, successful aphid transmission has occurred under experimental conditions and the possibility of weed reservoirs can not be ruled out. Broad-leaf weed management is known to be beneficial in the control of certain plant viruses such as ToRSV. Therefore, while the destruction of potential reservoir hosts is not a proven strategy, the elimination of wild Prunus and broad-leaf weeds in and near Prunus orchards may be of some value in preventing the reintroduction of PPV to previously quarantined areas.
Another tool for virus control is vector management. This strategy works best for virus vectors that are not highly mobile such as nematodes. Aphid management for the prevention of PPV is problematic. This approach is probably only of limited value in some situations. Horticultural spray oil is known to inhibit virus transmission by aphids and could be used to protect young, non-bearing trees but since oil can have a negative effect on fruit production this may not be practical on older trees.
Lessons Learned About Preventing the Introduction of Exotic Pests
The appearance of PPV in Pennsylvania highlights the ever-present risks posed by virus diseases and serves to raise the level of awareness that all growers need to possess. Orchard sanitation and good cultural practices are key issues in the prevention of virus problems. Start with a clean orchard 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. However, as our experience with PPV illustrates, pathogens sometimes are introduced to new regions either by illegal or unwitting use of infected propagation material or by natural means.
Good management practices require growers to be familiar with common disease problems and their control. It is also important for growers to remain vigilant for anything unusual and bring it to the attention of experts who can identify the problem.