Plum Pox Virus was first found in North America in 1999 in several peach orchards in Adams County, Pennsylvania. Plum Pox is a disease that affects peaches, plums, apricots, nectarines, and almonds. The virus causes the fruit to develop blotches, which lower their value. The disease doesn't kill the tree, but eventually an infected tree will stop producing any fruit whatsoever.

But long before this happens, the infected tree is a great danger to nearby trees. It is a reservoir of disease, ready to spread rapidly. Plum Pox Virus is spread from tree to tree by tiny insects called aphids. The aphid picks up the virus as it feeds on an infected tree, and when it goes to a different tree, it releases virus from its feeding tube and infects the next tree.

While aphids are definitely the vectors that spread Plum Pox Virus from tree to tree and orchard to orchard, they don't fly well enough--or live long enough--to have flown from Europe or South America to southern Pennsylvania, especially without touching down first anywhere else in the U.S.

The leap of this disease from continent to continent was caused by human action. The virus was probably present in nursery stock that was imported to the United States. It was then unknowingly grafted onto a healthy tree or planted into the landscape. The aphids took it from there. And since it takes several years after infection for fruits to show any symptoms, the disease was already well established by the time it was detected.