PPV-D was the strain that was recently discovered in Pennsylvania. In natural settings such as orchards, PPV is spread over short distances only by aphids. Aphids are small insects that feed through modified piercing-sucking mouthparts on internal vascular tissues of plants. Therefore, the potential exists for aphid spread of PPV in Pennsylvania. The mechanism by which aphids transmit PPV is called nonpersistent transmission.
Aphids make two kinds of probes on leaf surfaces, test probes and feeding probes.
Rapid transmission of PPV occurs specifically during aphid test probes and not during the longer-lasting feeding probes. The virus can occur in high concentrations in leaf cells, and when the aphid test probes into an infected cell, some of the virus is sucked into the stylet where it can stick to the lining of the food canal in the center of the stylet and remain infectious there for several minutes or hours. If the aphid then flies to a healthy plant and initiates a test probe into a healthy epidermal cell, the virus carried on the food canal can detach and be squirted back into the healthy plant cell when the aphid expels the contents of the food canal before sucking up fresh cell contents for taste testing. It is during this process that the virus is transmitted. Each aphid must feed directly on an infected plant, acquire sufficient virus, and then fly immediately to the next plant to effect a transmission.