2007 Plum Pox Survey
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2007 Plum Pox Survey
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Plum pox is a viral disease of stone fruits first reported in Bulgarian plums in 1915. More widely known around the world by its Slavic name, Sharka, the disease first spread slowly through eastern Europe, gaining momentum in the 1950s as it reached Germany, Poland and Russia. In 1970, the Plum pox virus was reported in France, and in 1986 it was reported in Spain.
Plum pox is considered the most widespread disease of stone fruits in Europe, with an estimated 100 million trees infected. Movement of the disease has continued, with symptoms first detected in the Western hemisphere in Chile in 1992. Plum pox has been detected in North America in Canada and in the United States in Pennsylvania, New York, and Michigan. In New York, the Plum pox virus was identified on two plum trees in Niagara County in July 2006, followed by a peach tree that tested positive in August of 2006.
Plum Pox follows the convention that viruses are named according to the plant host in which they are first discovered, hence the name Plum Pox Virus (PPV). Plum Pox Virus can infect all cultivated stone fruit species including peach, nectarine, apricot, almond, and cherry, as well as wild and ornamental Prunus. Spread of the virus is via aphid vectors and through infected budwood. New York infections are believed to be the result of aphid transfer of the virus from Canada. Six strains of PPV have been identified; all occurences in North America thus far have been identified as strain "D". Strain "D" has been shown to naturally infect peach, apricot, plum; almond and sweet and sour cherry are not naturally infected but can be artificially infected.
Newly infected trees are rarely symptomatic. It is critical that symptomless trees be regarded very seriously as they will act as a silent source of further infection. Symptoms of PPV occur sporadically and often are not apparent until three or more years after infection. Symptoms of PPV vary with the strain of the virus, temperature, and by cultivar, age, and nutritional status of the host. The virus may be detectable in some parts of the host but not others. PPV reduces the quality of stone fruit and eventually renders the tree useless for fruit production; even symptomless trees will produce reduced quantities of fruit. All infected trees, even if symptomless, are sources of possible further PPV transmission to other neighboring stone fruit orchard crops or susceptible Prunus hosts.
Leaf symptoms : Vein yellowing, light green to yellow rings; symptoms may be very subtle. Plum leaves are most severely affected, with necrotic brown or yellow spots often developing into shot hole. Peach leaves may show leaf crinkling, puckering, or curling.
 photos by M Cambra (ivia, Moncada, Spain) |
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Flower symptoms : peach varieties with showy blossoms may exhibit color breaking
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Fruit symptoms : Lightly pigmented rings or line patterns resulting from several overlapping rings may be apparent on the surface of the fruit. Apricot and plum fruit may be deformed, turn brown or become necrotic and may have rings on the surface of the seed.
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 photos by M Cambra (ivia, Moncada, Spain) |
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Short distance spread is the result of nonpersistent aphid transmission. When an aphid test probes a leaf cell, it penetrates the epidermal cell, expels any contents of the stylet into the plant and then draws fresh cell content to taste test the plant. During tasting of an infected host, some of the virus particles can be pulled into the stylet, stick to the lining of the food canal, and be transferred to a healthy plant. The virus does not persist in the aphid after it has been expelled into new tissue.
Long-distance spread of PPV occurs primarily by movement of infected plants or plant parts. Infection can spread through infected nursery stock or infected buds that are grafted onto healthy trees. Long-distance spread by aphids is unlikely as the lifespan of the virus within an aphid is generally less than an hour.
Exclusion
Commercial growers and nursery propagators should only purchase planting stock that is certified virus-tested after it has been assayed for PPV and other viruses. Almost universally, intercontinental spread of PPV has been associated with human transfer of infected materials. Therefore, once current, localized US outbreaks of PPV are under control, reliance on regulation and inspection combined with education of importers and travelers will be key in preventing the reintroduction of PPV to the US. Careful selection of budwood and limited exchange of budwood to avoid virus-infected material is critical in avoiding human transferrence of the disease.
Eradication
The timely elimination of any infected trees found in the US is a critically important control method for PPV. There is no treatment for PPV; once a tree has been confirmed to have PPV it should be removed as quickly as possible to limit spread of the virus to neighboring hosts. If tree stumps are left behind they should be treated with herbicide to avoid the development of sucker shoots, which can also carry PPV. Newly infected orchards are most often detected within 200 yards of an infected host, but can be range up to 1,000 yards from the host. Second season infections can be widely scattered from the original infecion site if not controlled. Recommendations are that growers wait three years before replanting to stone fruit after a PPV outbreak.
Insecticides
Aphid control is not a feasible management tool; application of insecticides may reduce the overall population of aphids over a growing season, but a single aphid can transmit PPV to a new host in a matter of seconds and total control of aphids is impossible to achieve.
Resistance
Limited naturally occuring resistance genes are available for use in developing
highly resistant stone fruit cultivars. Currently, the most promising prospect
for PPV resistance are genetically engineered resistant stone fruit species.
The insertion of PPV genes into plum or apriocot trees confers high resistance
or immunity to PPV. This resistance is heritable and can therefore
be incorporated into other cultivars through standard breeding practices.
Although this method is still in early, experimental stages, it offers hope
for the future development of commercially viable stone fruit crops that are
resistant to PPV.
Once leaf samples are brought to the lab at NYSAES, we test for the presence of PPV using an Enzyme Linked ImmunoSorbent Assay (ELISA). In very simplified terms, if the virus is present it will be captured by antibodies specific to Plum pox virus. The antibodies are treated with a series of reagents, with the end result that any sample containing PPV will undergo a color reaction that is detected by a scanner.
Any samples that test positive for PPV will be tested again by ELISA, followed by molecular techniques to verify the presence of the virus. If these tests are positive, samples will be sent out to two other labs for further confirmation prior to any action being taken.
Sampling of trees and all public outreach will be handled by the New York State Department of Agriculture and Markets. Surveys will be most tightly centered around orchards that had positive trees last summer. Within 500 m of tree sites that tested positive, all stone fruit trees will have two samples taken. Out to 1.5 miles from former sites of infection, every second tree will have one sample taken. Beyond this radius, sampling will be random throughout the state. Each sample consists of eight leaves taken from multiple locations on the tree, as distribution of PPV in infected trees is not uniform and this increases the probability that the virus will be detected if present.
Samples will be placed in plastic bags, bar-code labeled, and stored on ice until delivered to the NYS Agricultural Experiment Station in Geneva. Once the samples reach Geneva, they are scanned into a database and tracked by their barcode number. No information other than the barcode will be available to NYSAES.
If you have a question about the stone fruit survey for Plum pox virus , you should contact the New York State Department of Agriculture and Markets.
If you suspect that you may have an infected tree, contact your local extension agent as soon as possible.
