Plum pox virus surfaces in Pennsylvania orchards
Another Stone Fruit Virus Arrives in US
It took nearly 100 years, but now it has arrived.
After slowly spreading across Europe and leaping the South Atlantic to land in Chile, the crop crippling plum pox virus (PPV) has reared its ugly head in Adams County, Pennsylvania. Also known as sharka virus, the disease infects all species of commercially cultivated stone fruit, including almond, apricot, cherry, peach, nectarine and plum, plus most ornamental and native species of Prunus. PPV is considered an especially serious disease, with crop damage ranging from poor fruit quality to complete crop loss. There is no treatment, and control is limited to spraying for aphids that spread the virus, destruction of infected trees, and replacement with certified virus-free trees. The seriousness of the potential threat to California crops prompted a recent alert from the California Department of Food and Agriculture, the formation of a team of department scientists, and development of an action plan to monitor the situation and respond to the discovery of the virus in California...if, and when, that occurs. Meanwhile, CDFA is keeping in close contact with the USDA and the Pennsylvania Department of Agriculture to follow their efforts to track and eradicate the pest.
Considering the porosity of state and national borders these days, it might be wise to become better acquainted with PPV, in the off-chance California growers eventually have occasion to see the beast up close and personal. The following insights into the disease and its behavior were taken from material provided by Conrad Krass, primary state plant pathologist/nematologist with the Plant Health and Pest Prevention Services at CDFA. PPV was first reported in plums in Bulgaria in 1915. From there it spread slowly through eastern Europe. After 1950, however, it spread rapidly throughout the continent, reaching England in 1965, where it was eradicated, briefly, but reappeared in 1970, the same year France reported PPV. Spain found PPV in 1984. The disease continues to spread toward Eurasia and along the Mediterranean coast of Africa. In 1992, PPV appeared in Chile. By 1996, it had infected all stone fruit growing areas of that country. Eventually, it reached the US, showing up in Adams County, Pennsylvania, in October 1999.
How Serious Is It?
In Europe, PPV is considered the most devastating disease affecting stone fruits. An estimated 100 million European trees are infected. Once PPV becomes established, it is difficult to eradicate and takes a terrible toll. However, it is hoped that the relatively small known infestation in Pennsylvania can be isolated and eradicated, if it has not already spread beyond that area. It is believed that PPV spreads within orchards from infected to healthy fruit trees. The role of alternate weed hosts, if any, is not known. However, the possibility of weeds acting as alternate survival hosts for the virus still exists and needs to be examined.
Unfortunately, many trees fail to show symptoms for the first few years following infection by PPV. In Pennsylvania, for example, surveys to detect PPV found 18 infected orchards, but only two of the 18 had trees showing obvious symptoms. Therefore, symptoms are not a good indicator of infection and cannot be relied upon to determine the incidence or range of the disease. Symptoms induced by PPV also vary considerably by plant species, cultivar, age of plant, nutrient status of the plant, and environmental conditions. Also, different strains of PPV vary in virulence, which results in different degrees of disease severity. In general, however, symptoms on leaves may consist of mild light green discoloration bordering the leaf veins or chlorotic light green or yellowed rings. These symptoms may be obvious or barely visible to the eye, depending on the factors noted above. In fruit trees, the fruit may become discolored, deformed or irregular in shape, and drop prematurely from the tree. The infection reduces the quantity of the crop, lowers fruit quality, and eventually debilitates the tree, shortening its useful life. It may also affect the tree's response to insect and fungal attacks, cultural practices such as fertilization, and infection from other viruses. The plum pox virus species is in the genus Potyvirus. Potyviruses are one of the largest groups of plant viruses and infect a wide range of agricultural crops. A protein shell protects the PPV virus from degradation, which enables it to spread throughout a plant once it is introduced, infecting all tissues, including leaves, flower parts, buds, young bark and roots. PPV occurs in four known strains: PPV-D, PPV-M, PPV-C, and PPV-EA. The most common European strains are PPV-D and PPV-M. Symptom severity and pattern of spread are different for each. The PPV-D strain is the one infecting North and South America, which is fortunate, because it is not known to be readily seed-transmitted, but is spread more slowly by aphids. This gives hope of successful eradication if infected trees are quickly detected and destroyed.
How PPV Gets Around
In natural settings, such as orchards, PPV is spread only by aphids. Of the more than 2,800 species of aphids, only 12 to 15 species are known to transmit PPV. However, one of the most efficient vectors, the Green Peach Aphid, is prevalent in Pennsylvania. Aphids transmit PPV by probing into an infected plant and acquiring the virus, then moving to another plant, probing it to see if it is a plant that it likes to feed on. It is during these test probes that the PPV virus is transmitted. PPV viruses sucked up by the aphid from an infected plant live only a few minutes to maybe an hour in the aphid's probe. After that, the viruses can no longer infect another plant. Once the aphid makes a test probe, the virus is lost and will not be available for the next test probe. Consequently, an aphid cannot feed a single time on an infected plant and then transmit virus to several healthy plants. PPV does not replicate in the aphid and does not circulate in the aphid's body. In many cases, aphids are thought to spread PPV from leaf to leaf or branch to branch while test probing on a single tree, which results in multiple infections on one tree.
Obviously, the limited ability of aphids to transmit PPV can't account for the disease's spread across vast geographic distances. That is left to mankind. Long-distance spread of PPV is primarily by movement of infected plants or plant parts by people. Discovery of PPV in several European countries has been associated with introductions of infected nursery stock from other countries, including seedlings, root stocks and budding material. The appearance of PPV in Pennsylvania serves to remind everyone of the importance of strict plant quarantine and testing of imported nursery materials. One of the problems facing Europe in controlling the spread of PPV has been the implementation of the European Union and the subsequent elimination of border crossing inspections. This makes quarantines more difficult to enforce. Quarantine can be effective in preventing long-distance spread of PPV, especially when the disease is confined to a small area. A local quarantine preventing movement of infected materials out of the area was implemented October 21, 1999 for Huntington and Latimore townships in Adams County, Pennsylvania. If the quarantine went into effect before any infected material left the area, it might be effective in preventing the spread of the disease. The second strategy for preventing virus from being introduced 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.
Once an orchard is infected, the only strategy left to prevent further spread of PPV is to eliminate the infected trees as quickly as possible. A single infected tree in an orchard would serve as a virus reservoir and a source for all surrounding trees and adjacent orchards. It is necessary to destroy PPV-infected trees, including the roots. PPV can survive overwinter in various parts of infected trees. Rapidly growing sucker shoots developing from infected roots are known to be good sources of PPV. Blue Diamond® will follow developments in Pennsylvania and keep in close contact with CDFA, and will provide updates on the PPV issue when important news occurs.