Species infected include peach, nectarine, apricot, almond, and cherry, and ornamentals, such as flowering almond and purple leaf plum.
Plum Pox Virus is 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 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 barely visible to the eye, depending on factors described above. Flower symptoms can occur on varieties with showy 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.
Infected trees may or may not produce visual symptoms on leaves and fruits, but crop yield may be reduced even on symptomless trees. PPV also reduces fruit quality, resulting in reductions in grade, and eventually debilitates the tree, shortening its productive life. PPV symptoms may vary considerably with the cultivar, age, nutrient status of the host plant, and the temperature. In addition, different strains of PPV 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.
Mechanisms of PPV transmission and spread
Short-distance spread in and between orchards
In commercial settings such as orchards, PPV is spread over short distances by aphids. 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 is the green peach aphid (Myzus persicae), which colonizes peaches and other stone fruits in Pennsylvania.
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. Buds taken from infected trees will carry the virus and transfer it when grafted to healthy trees.
Identification and eradication of PPV in Pennsylvania
The plum pox virus is an introduced pathogen and given quarantine status by USDA/APHIS. After its discovery in Pennsylvania in 1999 an aggressive eradication program was developed to prevent it from spreading and eliminate it if possible. The eradication program included surveys to identify infected trees, destruction of infected orchards, creation of buffer zones and a moratorium on replanting Prunus in quarantined areas, among other efforts. After ten years and the destruction of over 1,500 acres of fruit trees, Pennsylvania was declared free of PPV in October 2009 and the moratorium on replanting Prunus was rescinded across the state.
Although the eradication program was successful, the occurrence of PPV in Pennsylvania serves to remind everyone of the importance 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 such as PPV from threatening U.S. crops. The Pennsylvania Department of Agriculture (PDA) will continue to monitor for PPV on a reduced scale for an additional 10 years to ensure that reservoirs of the virus did not escape detection. In addition, growers that spot suspect fruit are urged to bring it to the attention of PDA for testing.
Once PPV becomes established in a geographical region, it is very difficult to control or eradicate. Therefore, the primary focus is placed on preventing the introduction of PPV to new fruit-growing areas. Commercial growers and nursery propagators are reminded 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.
More information, including photos of PPV symptoms, can be found on the Sharka/Plum Pox section of this site.