First plum pox turns up in North America.

The U.S. Department of Agriculture is weighing whether to declare an official emergency in response to the first recognized North American outbreak of a dreaded fruit-tree virus.

Plum pox attacks much of the genus Prunus, including plums, peaches, nectarines, apricots, and almonds. Initially, the virus renders fruit too spotted or misshapen to sell. As the infection progresses, trees stop bearing any crop at all. Ornamental trees as well as those grown for fruit may catch the virus.

All orchardists can do is bulldoze and burn sick trees in hopes of slowing pox spread. The virus doesn't affect people's health but plays havoc with their pocketbooks.

Described early this century in eastern Europe, plum pox has spread through much of Europe and also turned up in India, Egypt, and Syria. In 1992, growers found it in Chile, but North America escaped--until this fall. On Oct. 20, USDA and the Pennsylvania Department of Agriculture announced finding plum pox in Adams County, which includes Gettysburg.

"We're looking at the number-one disease of stone fruits--it's devastating," says Gary Clement, plant-health director for Pennsylvania in the federal Animal and Plant Health Inspection Service (APHIS) office in Harrisburg.

The disease spreads when people graft or move infected plants and when any of a dozen or so aphid species pick up virus on their mouth parts and inject it into the next plant they probe. Aphids carry the virus for a few hours at most, and it doesn't stick to other insects. So, Clement says he suspects plum pox reached the United States on a smuggled plant.

The virus has probably been in Adams County for several years, notes John M. Halbrendt, a pathologist for Pennsylvania State University in State College. A grower had noticed spots on a few peaches last year but couldn't find the cause.

This fall, a fruit-packing operation alerted Ruth Welliver, a state plant pathologist, to spotted peaches. She tested leaf samples against an array of antibodies that react to any of the huge group of nightmare pathogens called potyviruses. Several hundred bedevil crops from potatoes to papayas, but plum pox is the only one known to attack stone fruits.

Welliver's potyvirus test gave the first positive result she'd ever seen for a stone fruit. She sent a picture of the suspect peaches to one of the few U.S. scientists who's worked with plum pox. Seeing the ominous splotches, "I just about died," remembers Laurene Levy of APHIS in Beltsville, Md.

She, Welliver, and USDA's Vern D. Damsteegt at Fort Detrick, Md., worked around the clock running confirmatory tests. Levy identified the outbreak as strain D, which she explained doesn't attack cherries and doesn't spread through seeds. "That was the only good news I could give them," Levy says.

Plum growers may also take comfort from a resistant tree, C-5, that a few forward-looking breeders have been developing for more than a decade. Its resistance comes from a bit of the virus' genome that researchers inserted into the tree's genetic material.

The big question, whether growers caught the pox virus early enough to eradicate it, remains unanswered. However, Clement says, "I'm optimistic."