<p style="text-align: justify;">Plum pox, also known as sharka, is the most devastating viral disease worldwide of stone fruit including peaches, apricots, plums, nectarines, almonds, and sweet and tart cherries. The disease significantly limits stone fruit production in areas where it is established. More than 100 million stone fruit trees in Europe are infected. First described on plums in Bulgaria in 1915, plum pox has spread to a large part of Europe, the Mediterranean, the Middle East (Egypt and Syria), India, and Chile. In 1999, for the first time in North America, plum pox was detected in a Pennsylvania orchard.</p>

<p style="text-align: justify;">The virus causing the disease is plum pox potyvirus (PPV). PPV infects various members of the genus <i>Prunus</i>, such as the stone fruit trees mentioned above. Wild and ornamental species of this genus may also become infected by some strains of the virus. Some weed hosts identified in the field and numerous hosts tested in laboratory settings have also been known to become infected with PPV.</p>

<p style="text-align: justify;">Plum pox is economically important because it can cause fruit to be unmarketable and can decrease the yield of infected trees. The severity of the disease depends on the strain of the virus present and the susceptibility of <i>Prunus</i> cultivars (cultivated varieties of plants). The presence of PPV can enhance the effects of other endemic viruses infecting various species of the genus <i>Prunus</i>, such as prune dwarf virus, Prunus necrotic ringspot virus (causes browning), and apple chlorotic leaf spot virus (causes yellowing), resulting in still greater economic losses. In southeastern France, a newly identified strain of PPV induces severe necrosis, resulting in early leaf drop and tree decline, even in the absence of endemic <i>Prunus</i> viruses.</p>

<p style="text-align: justify;">Plum pox is spread from plant to plant by several aphid vectors (insects that suck sap from plants then carry the virus to other plants) and is also transmitted in budwood used for grafting. Studies indicate that at least 14 aphid species can transmit PPV. These species include <i>Myzus persicae</i>, <i>Aphis spiraecola</i>, <i>A. gossypii</i>, and <i>A. fabae</i>. Some PPV strains have been identified that are not transmissible by aphids. Control and prevention measures for PPV include field surveys, use of certified nursery materials, use of resistant plants (when available), control of aphids, and elimination of infected trees in nurseries and orchards.</p>

<p style="text-align: justify;">Sources of resistance exist in <i>Prunus</i> but are not abundant and, for this reason, a genetically engineered PPV-resistant plum has been developed using the coat protein gene from the virus itself. Multiple insertions of the gene have been identified in the plant, but no coat protein is produced by these genes. The resistance is due to RNA silencing, where the expression of a transgene induces the plant to degrade RNA of the same sequence, including any RNA from an infecting virus, thus preventing infection.</p>

<p style="text-align: justify;">In addition to the use of the engineered variety for direct production, the resistance can by transferred through hybridization to other plum trees. This provides a unique source of germplasm for future breeding programs worldwide as there are few sources of resistance to this disease in conventional germplasm and they have proved difficult to introgress into cultivated varieties. In the United States, there are no plans at this time (2007) to release the transgenic varieties for commercial use as the disease is being controlled though quarantine practices, however discussions with breeders have initiated to develop plans for the deployment of this trait should the need arise.</p>
