In October 1999, the presence of plum pox was confirmed in Pennsylvania, with a press release from the Pennsylvania Dept. of Agriculture, and communications from George Greene of the Penn. State Fruit Lab at Biglerville. Further information was provided by USDA-APHIS-PPQ in Richmond on October 27. The following update contains information from these sources, as well as information presented in the West Virginia, Penn State and Virtual Orchard web sites.
The presence of the disease was confirmed on October 13, 1999, in a block of Encore peaches in Adams County. A survey was carried out in orchards within one mile of the infected block; 10 blocks with plum pox were found, belonging to three different growers, including cases in peach, nectarine and plum. The search was expanded on October 21-22 to an additional one-mile radius. No symptoms were detected, though leaf analyses are pending. A map of the quarantined area is available on the web. This is the first case of plum pox in North America, although it was introduced into Chile in 1995, and caused concern for North American specialists at that time. The disease has since been found in Cumberland County PA and in Canada. Surveys in Virginia and Michigan have been negative. 2002 survey samples are sent to Agdia.
Background of Disease: Most stone fruits are susceptible, with the exception of cherries. Trees are not killed but infected fruit are unmarketable because of spots and ring blemishes. Fruit may also drop prematurely from the tree. The disease is spread over short distances via nonpersistent aphid transmission and over larger distances through the movement of infected budwood or nursery stock (nurseries that served as sources of trees for the affected orchards are being contacted, but so far there is no evidence any were involved). Normal movement of people and equipment from infested orchards is not thought to play a role in the spread of the disease. More details on the biology of the disease are included in the APS Compendium of Stone Fruit Diseases by a British Plant Pathologist, A. N. Adams, and in other international sites. Alan Biggs has posted an excellent photo of symptoms on an Encore peach (see above, courtesy of West Virginia University), as well as a more extensive gallery of plum pox photos.
The strain of the virus present in Pennsylvania has been identified as the D strain. This strain is present in western Europe and is described as being less aggressive than some other strains (including a reduced ability to be transmitted by aphid vectors) and known to be not seed transmitted. Control is by use of virus free plants, and aphid control. The latter approach, used in Europe will need to be critically evaluated in the mid-Atlantic region if aphids are less likely to be vectors than in the Old World.
Role of aphids: Aphids are involved in the transmission of plum pox. The specific strain of the disease in the Pennsylvania outbreak (strain D) is less likely to be transmitted by aphids. Further work will therefore be needed on the prospects of management by vector control. Brunt et al. (1996) reported the main natural vectors as Brachycaudus helichrysi (leaf-curling plum aphid, related to the native black peach aphid), Myzus persicae (green peach aphid), and Myzus (Phorodon) humuli (damson-hop aphid). Additional aphids reported as vectors include Aphis craccivora (black legume aphid or groundnut aphid), A. spiraecola (spirea aphid), B. cardui (thistle aphid, overwinters on plum), and M. varians (peach leaf-roll aphid).
Not all sucking arthropods are capable of transmitting the disease, e.g. some aphids (Myzus cerasi (cherry aphid), Hyalopterus pruni (mealy plum aphid)), leafhoppers (Edwardsonia plebeja, Macrosteles laevis), spittle bugs (Aphrophora alni), soft scales (Lecanium corni or European fruit lecanium), plant bugs (Lygus pratensis (=lineolaris), the tarnished plant bug), and mites (Paratetranychus pilosus) were reported to be unable to transmit the virus.
Most of the listed vector species are reported on peach in the U.S. by Blackman and Eastop (1984, 1994) and Stoetzel and Miller (1998). Of the main aphid vectors, the green peach aphid is a common pest of peach in our region. B. helichrysi, originally European, is now worldwide in distribution and widespread in the U.S. The damson-hop aphid overwinters on plum and some other Prunus species, and is widespread in the U.S. (Blackman and Eastop 1994). Of the lesser aphid vectors, A. craccivora has a wide host range and is widespread globally (but is not on the list of peach aphids given by Stoetzel and Miller (1998)). Spirea aphid, has become more prominent on apple and citrus in recent years, almost totally displacing apple aphid on apple, and colonizes peach as well. The thistle aphid overwinters on plum and a few other Prunus species. M. varians overwinters on peach where it has as holocyclic life cycle. It is mainly an Asian species, has been introduced into North America, but apparently here it is anholocyclic, occurring only on secondary (non-Prunus hosts. The black peach aphid, Brachycaudus persicae, is not reported to be a vector of any viruses, but since at least two other species in the genus can transmit plum pox virus, and this aphid may be a pest in our region, this should be reevaluated.
Transmission: Plum pox is transmitted in a non-persistent manner. This term should be explained. In persistent transmission of plant viruses, acquisition or transmission of the virus increases with increased feeding time. There is also a latent period before the aphid can transmit the disease that may be 12 hours or more. The aphid remains infective for a long time, possibly over its whole life, though transmission efficiency declines with age. Infectivity is not affected by molting, important because the virus is not lost as aphids mature (foregut and hind gut is lined with cuticle that is shed with the molt; viruses transmitted in a persistent fashion must be either in midgut or within the body).
Non-persistent transmission is more common in the aphids. Acquisition and transmission is not related to feeding time; the virus can be picked up or transmitted after only short feeding periods. The virus is carried on or in the stylets, and so is lost at the aphid molt.
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