Short distance spread of Plum pox virus (PPV) is the result of aphid transmission in a nonpersistent manner.  Aphids test leaf and fruit surfaces by probing them.  When an aphid test probes a leaf or a fruit cell, the aphid’s sap-sucking mouthpart, named stylet, penetrates the tissue and draws up cell contents.  Test probes last as little as 30 seconds.  During tasting of an infected host, virus particles can be pulled into the stylet and stick to the lining of the food canal.  Once acquired, PPV remains in the stylet for up to three hours.  During this time the virus can be transferred to healthy trees when viruliferous aphids expel their stylet contents during new probes.  The virus does not persist in the aphid after it has been expelled into new tissue.  Spring aphid flights are important for spread within and between orchards.  Several aphid species can transmit PPV but only a few of then are considered important vectors in the northeastern US: the black bean aphid (Aphid fabae), the spirea aphid (Aphid spiraecol), the black peach aphid (Brachycaudus persicae) and the green peach aphid (Myzus persicae).  All infected trees, even when not showing symptoms, are sources of possible PPV transmission to healthy trees.  Aphids can spread PPV from several yards to a few miles and is unlikely to occur over long-distance as the lifespan of the virus within an aphid is generally less than an hour.
Long-distance spread of PPV over several miles occurs primarily by movement of infected plants or plant parts.  Virus infection can spread through infected nursery stock or infected buds collected from infected trees.
Spatial analysis of PPV-infected trees in orchards suggests a preferential virus spread several tree spaces away from infected trees, rather than to neighboring trees. Thus, secondary infections can be widely scattered from the original infection site if the primary virus sources are not controlled.

Aphid vectors of Plum pox virus: the black bean aphid (Aphis fabae, left) (photo by B. Nault) and the green peach aphid (Myzus persicae, right) (photo by J. Ogrodnick) are major vectors.