- Plum Pox Virus Detected in
Canada and Another
Pennsylvania County
Managing Editor
The October discovery of plum pox, the first such confirmed finding of the disease in North America, took place in two townships – Latimore and Huntington – in Adams County, Pennsylvania. Peach and nectarine trees in those two townships were quarantined, and a massive nationwide survey of trees launched in an attempt to stop an epidemic in its tracks.
However, plum pox virus, which strikes stone fruit trees, has popped up outside the original quarantine area. The Canadian discovery was announced June 23. The disease was found in a nectarine orchard near Niagara-on-the-Lake, Ontario.
The new Pennsylvania discovery was made in Cumberland County, in an orchard that straddles Adams and Cumberland counties. That took place in early July, and caused South Middleton Township to be added to the quarantine list.
In each case, the strain of the disease is the same, strain D. This is a relatively weaker strain that does not affect cherries, but does strike peaches, nectarines, apricots and plums. As a response to the initial outbreak, the Canadian Food Inspection Agency (CFIA) in November 1999 banned importation of prunus (stone fruit) from the United States. The affected nectarine trees in Canada were imported from Adams County, according to the CFIA.
Survey teams are making thousands of visits across the United States, inspecting stone fruit orchards and adjacent areas. The goal is to determine the incidence of the virus and then come up with strategies to wipe out the virus.
Samples are taken and sent to laboratories for testing. The survey is complete of all susceptible nursery stock propagated and grown in four Pennsylvania counties. The number of blocks with positive responses was at 37 as of the first week of July.
In New York state, a team planned to survey 9,050 nursery trees, 4,560 orchard trees and 1,500 foundation trees of plums, peaches and apricots. A similar effort is underway in Michigan with the focus on surveying trees used for budwood and on orchard trees near nurseries.
The virus, also known as sharka, was first discovered in Bulgaria in 1918. It is now widespread in Europe, moving from former Eastern Bloc nations westward to Spain, Italy, France and the Netherlands.
Symptoms vary among different species, while some plants show no symptoms at all. Peach leaf symptoms include pale green spots, rings and lines that are visible from early summer onward. These markings can become dead spots (necrotic). Infected trees are usually not stunted and are thus difficult to identify. In peaches, symptoms appear in the first leaves to expand and consist of chlorotic vein clearing and vein banding, with twisting and distortion of the leaf.
Peach fruit develop pale or dark rings, lines and spots. These symptoms may disappear at ripening. In plums, fruit symptoms consist of rings and blotches in fully expanded unripe fruit. These markings usually disappear as the fruit colors and reaches maturity. Sunken rings and spots occur in some varieties. Red rings and spots can occur on the stones.
Plum pox is a member of the potato virus Y group. It is transmitted by grafting infected budwood or by aphids. Seven of the eight aphid species that can vector the virus are found in Pennsylvania. The insects pierce the fruit or leaf with their sucking mouthparts and transmit the virus. The D strain of the virus cannot be transmitted by seed.
For more information on plum pox virus, visit http://sharka.cas.psu.edu and http://state.pa.us/PA_Exec/Agriculture/plum_pox/
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