Last summer, a Pennsylvania grower in Adams County recognized unusual symptoms on Encore peaches. After a series of laboratory tests in October 1999, USDA scientists positively identified the problem as plum pox. Plum pox, also known as sharka, is the most devastating viral disease of stone fruits. Plum pox virus (PPV) was first described in Bulgaria in 1915. The virus subsequently spread to a large part of Europe, the Mediterranean coasts, the Middle East, India, and Chile.
During October 1999, a survey was conducted in Pennsylvania to determine the extent of disease distribution within a 2-mile radius from the initial positive orchard block. In addition, surveys were carried out in and around Adams County nurseries, in orchards adjacent to the nurseries, and some other Encore peach orchards. Of the 218 orchard block samples tested, PPV was found in 18 orchard blocks, of which only two showed the viral symptoms. The virus was found in both peach and plum samples. The Pennsylvania Department of Agriculture placed a quarantine on the two Adams County townships on November 6,1999, prohibiting the movement of budwood or trees from the area. Because there is no evidence that the virus is spread from the fruit, there has been no restriction on sale of fruit from the area. All 18 orchard blocks that tested positive for PPV, will be removed.
Illinois Extension is participating in a nationwide survey of stone fruit nurseries and orchards for plum pox virus and has collected more than 3,000 leaf samples from stone fruit trees. To date, all Illinois samples have tested negative for PPV.
Host Range and Economic Importance: PPV infects stone fruit trees including peaches, nectarines, plums, apricots, cherries, and almonds. In addition, wild Prunus species and a large number of weeds were infected under laboratory conditions. There are approximately 2,500 acres of orchards, 10,000 budwood trees, and 500,000 nursery stock trees of Prunus species—peaches, nectarines, apricots, plums, cherries, and almonds—in Illinois.
Plum pox is economically important because it reduces fruit yield and quality. Infected fruits may be unmarketable. The severity of the disease depends on the susceptibility of the host and the strain of the virus. More than 100 million stone fruit trees in Europe are reported to be infected with PPV.
Symptoms and Spread: Symptoms of plum pox on stone fruit trees vary with the plant species, the cultivars, age and nutrient status of the plant, the strains of the virus, and the temperature. Some infected plants may not exhibit symptoms at all. However, serological tests can detect PPV before visual symptoms develop. Symptoms may include dark pink stripes on the flower petals, light green discoloration bordering the leaf veins (vein yellowing) or yellow to light green spots or rings, lightly pigmented yellow rings or line patterns on the surface of the fruit, deformed fruit, dark rings or lesions on the fruit skin, and internal reddening of the fruit. Symptoms frequently are restricted to only a few leaves per shoot. Plums generally are more severely affected and show more severe symptoms than other stone fruits. Therefore, they are a good indicator host to observe for symptoms of infection. Studies have indicated that 14 aphid species, including the green peach aphid (Myzus persicae) transmit PPV.
To date, PPV has been reported only in Pennsylvania and no spread of the virus to other areas in the United States has been confirmed. Introduction of PPV to Illinois could be devastating for stone fruit production. It is essential to observe the quarantine regulations placed by USDA–APHIS and Illinois Department of Agriculture. If PPV becomes established, it is very difficult or impossible to eradicate. Planting only certified disease-free trees will avoid introducing this pathogen. Update information on PPV is available on a Web site maintained by Pennsylvania State University (http://sharka.cas.psu.edu).
(Adapted from Illinois Fruit and Vegetable News [March 3, 2000, Vol. 6, No. 3; and May 4, 2000, Vol. 6, No. 10], written by Dr. Mohammad Babadoost [217-333-1523; firstname.lastname@example.org]).