[ Search PSU ]   

Special Section: Plum Pox

Sharka: Plum Pox Virus of Stone Fruits

Disease history and distribution
The disease referred to as plum pox was first reported in 1915 in Bulgarian plums. Sharka is the Slavic name for plum pox and is the most widely used common name for the disease around the world. The disease spread from Bulgaria northward through Eastern Europe, reaching the former Yugoslavia in 1935 and Hungary in 1941. It spread more rapidly after about 1950, reaching Germany in 1956, Poland and Russia in 1961, and France in 1970. In 1984, Spain became the most recent western European country to be invaded by plum pox.

Throughout Europe, plum pox is considered the most devastating disease of stone fruits. It has been estimated that over 100 million European trees are infected. Plum pox continues to spread eastward in Eurasia and southward along the Mediterranean coast of Africa. Sharka first appeared in the Western Hemisphere in Chile in 1992.

At this time, the only known occurrences of plum pox in North America are in Ontario, Canada, and in a small area of Adams, York, Franklin, and Cumberland counties in Pennsylvania. Because of the localized nature of this infection, it is hoped that eradication may be successful in eliminating these pockets of plum pox in North America.

Susceptible plants
Plum pox disease is caused by a virus. Plant viruses are named according to the plant host in which they are first identified, hence the name plum pox virus or PPV. PPV infects not only plum but also all other economically important Prunus species including peach, nectarine, apricot, almond, cherry, and ornamentals such as flowering almond and purple-leaf plum. PPV is also known to infect wild Prunus and a large number of native and introduced weeds under laboratory conditions. Some common plants that can become infected with PPV include lamb's quarter (Chenopodium spp.), shepherd's purse (Capsella bursa-pastoris), ground cherry (Physalis spp.), buttercup (Ranunculus spp.), red and white clover (Trifolium spp.), and sweet clover (Melilotus spp.). Common garden hosts include tomato, pea, petunia, and zinnia.

Symptoms
PPV infection may produce symptoms on leaves and fruits, but it may also reduce yield even on symptomless trees. In addition, it reduces fruit quality, resulting in reductions in grade, and eventually debilitates the tree, reducing its useful life.

PPV symptoms in stone fruits may vary considerably with the cultivar, age and nutrient status of the plant, and the temperature. Also, different strains of PPV may vary in the severity of the disease they cause and the resulting symptoms. Not every leaf or fruit on an infected tree will show symptoms. The virus can often be detected at the bottom of a branch but not the tip; however, once a branch shows symptoms, it will continue to display them in subsequent years.

Diagnostic symptoms on leaves may consist of mild light green discoloration bordering the leaf veins (vein yellowing) or yellow to light green rings. These symptoms may be only barely visible to the eye, depending on factors described above.

Flower symptoms can occur on varieties with showy peach blossoms, but do not always occur. Peach and apricot fruit may develop lightly pigmented yellow rings or line patterns resulting from several rings running together on the surface of the fruit. Fruit may become deformed or irregular in shape and develop necrotic or brown dead areas. Apricot fruit may show no external evidence of disease, but may have a white ring or line patterns on the seed.

Plums generally are more severely affected and show more severe symptoms than other stone fruits. Therefore, plums are a good indicator host to observe for symptoms of infection, allowing growers to monitor for PPV infection in orchards. For some plum cultivars, infected fruit drops prematurely from the tree. Infected plum fruits often are severely deformed and develop darker rings or spots on the skin and a reddish discoloration of the flesh. In Hungary, green shoot cankers can be observed in the fall on infected plums. The cankers are brown with purple borders.

Many trees fail to show visual symptoms for the first three years following infection; however, serological tests performed in the laboratory can detect PPV before visual symptoms develop. Serological surveys to detect PPV in Pennsylvania orchards, directed by Ruth Welliver at the Pennsylvania Department of Agriculture (PDA) in October of 1999, detected 18 infected orchards, but only 2 of the 18 had trees showing obvious symptoms. A lack of symptoms, therefore, is not a good indicator that the orchard is healthy. Symptoms alone cannot be relied upon to determine the incidence or range of the disease. When symptoms do occur, however, they frequently indicate the presence of PPV.

The first person to identify the symptoms of PPV in Pennsylvania peach orchards was a grower, and growers have the opportunity to see more trees over a longer period of time than anyone else. Thus the role of growers, extension educators, and homeowners in identifying stone fruit trees with PPV symptoms is important to the total eradication of the disease from the state. The continual vigilance of stone fruit growers will supplement the serological surveys conducted by the PDA. Since the possibility always exists that an infected tree has been missed, a grower who identifies a potentially infected stone fruit tree should contact the local county extension office to have the tree serologically tested for PPV.

Characteristics of plum pox virus
Plum pox virus is known to occur in several different forms or variants, called strains. At this time, four distinct major strains have been identified and designated as PPV-D, PPV-M, PPV-C, and PPV-EA. These strains can be distinguished using laboratory tests. The most common European strains reported are PPV-D and PPV-M. These two strains differ in symptom severity among host species and in patterns of spread by aphids. PPV-D was the strain that was recently discovered in Pennsylvania. This may be fortunate because PPV-D is not known to be readily seed transmitted and seems to be more slowly spread by aphids in Europe than PPV-M, thus giving more hope of successful eradication if infected trees are quickly detected and destroyed.

Mechanisms for PPV transmission and spread
Short-distance spread in and between orchards
In natural settings such as orchards, PPV is spread over short distances by aphids. Recent research has shown that at least six North American aphid species are able to vector PPV, and four of these are common in Pennsylvania orchards. One of the most efficient vectors was the green peach aphid (Myzus persicae), which colonizes peaches and other stone fruits in Pennsylvania.

The mechanism by which aphids transmit PPV is called nonpersistent transmission. This refers to the fact that once the aphid probes into an infected plant and acquires the virus, the virus remains infectious and can be transmitted by the aphid only for a short time (a matter of minutes). Aphids make two kinds of probes on leaf surfaces, test probes and feeding probes. Rapid transmission of PPV occurs specifically during aphid test probes and not during the longer-lasting feeding probes.

When the aphid test probes into an infected cell, some of the virus is sucked into the stylet where it can stick to the lining of the food canal and remain infectious for several minutes or hours. If the aphid then flies to a healthy plant and initiates a test probe, the virus can detach and be squirted back into the healthy plant cell when the aphid expels the contents of the food canal before sucking up fresh cell contents for taste testing. It is during this process that the virus is transmitted.

In addition, virus that has been picked up by an aphid is lost during the next test probe; therefore, aphids feeding on an infected cell can transmit that virus only to the next cell on which they feed. Each aphid must feed directly on an infected plant, acquire sufficient virus, and then fly immediately to the next plant to effect a transmission. Aphids are thought to spread PPV from leaf to leaf or branch to branch while test probing on a single tree, resulting in multiple infection sites on one tree. PPV does not increase in the aphid and does not circulate in the aphid's body.

The efficiency of PPV transmission or rate of virus spread from apricot to peach, from peach to peach, and from plum to peach may all differ.

Long-distance spread between orchards or geographical regions
Long-distance spread of PPV and the introduction of the virus to new regions where it previously has not been known to exist occurs primarily by movement of infected plants or plant parts by human activity. Discovery of PPV in several European countries has been associated with introductions of infected nursery stock from other countries. Buds taken from infected trees will carry the virus and transfer it when grafted to healthy trees.

Long-distance spread by flying aphids is unlikely even in continuous land areas because aphids lose the virus when they probe on any non-Prunus species. Also, PPV probably becomes noninfectious in the aphid within approximately an hour after acquisition. In Europe, newly planted PPV-free Prunus orchards located 500 yards from the closest PPV-infected orchards have been reported to remain virus free and healthy.

The role of weeds
One aspect of plum pox virus spread that is poorly understood is the potential role of native plants or weeds in PPV survival and spread. Laboratory tests in Europe have identified several dozen common plants or weeds that can be mechanically inoculated with PPV. The United States Department of Agriculture (USDA) is also involved in laboratory experiments to identify non-Prunus plant species that may become infected with PPV, mechanically or by aphids. Whether or not weeds could play a significant role in PPV survival and spread between orchards in Pennsylvania has not yet been determined. Research is currently being conducted in Pennsylvania in infected orchards and on homeowner properties near infected stone fruit trees to determine the incidence of PPV in weeds and landscaping plants. Surveys over the past 6 years have tested over 66,800 weed and native tree samples from more than 80 plant families. Wild Prunus trees (predominantly black cherry) were included in the surveys. At this time, no evidence has been gathered to show that weeds or wild Prunus are acting as reservoirs for PPV in Pennsylvania. Negative results over several years are needed to show that the virus is being effectively controlled in this area.

Control: identification and eradication in the United States
If plum pox virus is discovered in a growing area, the first control strategy is to eliminate the virus-infected trees as quickly as possible before the virus spreads. A single infected tree in an orchard would serve as a virus source for all surrounding trees and for closely adjacent orchards.

When PPV is detected in a new region, intensive surveys must be taken to identify the extent of its spread. The Pennsylvania Department of Agriculture (PDA) has engaged in statewide surveys for the past six growing seasons to locate orchards that may be infected. These surveys have encompassed testing of orchards, commercial nurseries, and homeowner properties where stone fruit is present. In the event that PPV-positive trees are discovered, both the pocket of infected trees and a buffer zone surrounding that pocket must be destroyed. Buffer zones have been established around infected areas to eliminate all exposed trees that may have latent PPV infections. Each buffer zone extends 500 meters beyond the perimeters of infected areas. A zone that extends from 500 to 1,000 meters beyond the perimeters of the area is subjected to concentrated assays, in which 100 percent of the stone fruit trees in that zone are tested.

Control: exclusion and quarantine in the United States
Once PPV becomes established in a geographical region, it is very difficult or impossible to eradicate. Therefore, the primary focus is placed on preventing the introduction of PPV to new fruit-growing areas. In the United States, this is the responsibility of the Animal and Plant Health Inspection Service (APHIS) of the USDA. All fruit nursery stock for importation is tested for a range of known fruit tree pathogens, especially those that are not known to occur in the United States ("exotic" pathogens). Only pathogen-free material is released for commercial use.

The occurrence of PPV in Pennsylvania serves to remind everyone of the importance of and the need for strict plant quarantine and testing procedures associated with imported nursery materials. In almost all cases, intercontinental spread of plant disease causal agents is associated with human transfer of infected host materials. Therefore, once the diseases have been eliminated, careful regulation and inspection, combined with education of importers and travelers, could prevent the reintroduction of exotic plant diseases from threatening U.S. crops.

Quarantine also can be effective in preventing long-distance spread of PPV within a region, state, or country. If the disease occurs only in a small area, it may be contained by local quarantines preventing movement of infected materials out of that area. PDA implemented such a quarantine on October 21, 1999, for Huntington and Latimore Townships in Adams County, Pennsylvania. Since that time, additional townships and boroughs have been added to the quarantine area as additional infected trees have been identified (see map).

The spread of PPV within Pennsylvania appears to be predominantly due to the movement of aphid vectors. Three years of national surveys conducted by USDA/APHIS have included 39 states. The only positive results for either year have been from Pennsylvania. An interesting point to note here is that the state of New York has not found PPV, even though parts of the state lie in close proximity to the areas of quarantine in Canada.

An integral part of the PPV quarantine program in Pennsylvania has been the moratorium on the planting of stone fruit trees. Until the moratorium has been officially lifted, no Prunus may be planted within the boundaries of the quarantine zone. The preclusion of new host trees should prevent the survival of the plum pox virus within those areas. The order is expected to remain in effect until three years have passed with no new trace of the virus.

The second strategy for preventing virus introduction into to a new area is for commercial growers and nursery propagators to purchase only certified virus-free planting stock that has been tested and verified to be free of PPV and other fruit viruses. In the future, it will be important for growers to verify that certified stone fruit nursery stock, from any source, also has been tested for PPV.

Control: protection from spread by aphids
At present, fruit trees cannot be completely protected from aphid inoculation with PPV. Insecticide applications can aid in reducing total aphid populations on fruit trees; however, it may take only one or a few aphids to inoculate a tree, and total or near-total aphid control is impossible to achieve.

In addition, the most efficient PPV aphid vector species may not even feed on or colonize stone fruits. Controlling this type of transmission is difficult because the aphids are so mobile. They need to probe for only a few seconds to acquire or transmit the virus, leaving little time to effectively apply a contact insecticide.

Also, controlling all potential aphid vectors requires scouting for different aphid species over the entire growing season. Some PPV vector species colonize peach only in early spring and then move to non-woody perennials during the summer; some aphids colonize peach during the growing season; and other species feed on peach only in the fall and lay eggs for overwinter survival on the trees.

Resistance: plant breeding and genetic engineering
If eradication of PPV from an area is not possible and if trees cannot be adequately protected from virus infection, then plant resistance to the virus is the only remaining viable control strategy. Few naturally occurring resistance genes are available for plant breeders to use in developing highly resistant fruit varieties.

Hybrid plum cultivars have been identified that respond to PPV by a hypersensitive response. This means that once virus infection occurs, the plant tissue surrounding the infection site quickly dies. PPV, like all other viruses, can survive and multiply only in living host cells. Therefore, the hypersensitive response prevents spread of the virus to other parts of the tree and eliminates the virus from the host. Unfortunately, the hypersensitive response in these plum hybrids is regulated by several different genes in the tree, making it difficult to incorporate the hypersensitive trait to other cultivars by standard plant breeding methods.

At this time, the most progress in developing resistance to PPV has involved genetic engineering of resistant stone fruit species by inserting specific PPV genes into stone fruit plants. For reasons that are not well understood, incorporation of the PPV genes into plum causes the plant to become highly resistant or almost immune to PPV. Similar work also has been reported for apricot in France. This type of resistance is inheritable and transmitted through seed and can be incorporated into other cultivars by standard plant breeding methods. Although this method is still only in the experimental stages, it is hoped that this type of resistance can be used to develop commercially viable cultivars of stone fruit crops.

Status: the current Pennsylvania plum pox virus situation
Results from PDA's state PPV surveys indicate that the eradication effort is meeting with success. As of December 2005, there were only five PPV-positive trees out of 213,000 tissue samples that were tested that year. The five positive trees came from just three orchards located in a relatively small section of the quarantine zone. The fact that the search for PPV-infected trees has intensified while the number of positive finds has been dramatically reduced suggests that eradication is likely in the near future.

For further information
More information, including photos of PPV symptoms, can be found at the following Penn State's Sharka Web site.