Viral epidemiologist Fred Gildow has spent his career tracking how viruses are transmitted to plants, and he sees plum pox virus as a new twist on a familiar theme. “Plum pox belongs to a very common family of plant viruses called potyviruses, which include maize dwarf mosaic virus and potato virus Y,” Gildow explains. “These plant viruses are very common in Pennsylvania and the United States.”

Potyviruses, including plum pox virus, are transmitted from plant to plant by aphids. At a USDA containment facility in Frederick, Maryland, Gildow and USDA plant pathologist Vern Damsteegt are studying which aphid species can infect fruit trees, whether aphids can pass the disease into other plants, and whether aphids spend their lives on a single peach tree or migrate from plant to plant and field to field. From previous work on other potyviruses, Gildow already knows how aphids transmit the plum pox virus into fruit trees.

Pennsylvania Department of Agriculture plant virologist Ruth Welliver tests samples from infected fruit trees within the quarantined area, which includes two townships in Adams County and two adjacent townships in Cumberland County.

“Aphids aren’t real smart,” Gildow says. “They have no idea what type of plant they’ve landed on. To see if they want to feed on it, aphids use a syringe-like mouthpart called a stylet to pierce the epidermal cell and ‘taste’ the plant’s sap.” It is during these testing probes that aphids pick up the plum pox virus. If the pox-carrying aphid is a finicky eater and flies away to test-probe another plant, the pathogen may be passed to the next plant if conditions are right. “Plum pox virus is only transmittable during the aphid’s test probe,” Gildow says. “For some reason the virus will not adhere to the stylet when aphids probe deep into the plant’s phloem, the circulatory system that carries nutrients and sugars.”

To determine which aphid species can transmit plum pox virus, Pennsylvania Department of Agriculture scientists are collecting aphid populations from around Adams County. Gildow will be identifying aphid species common to fruit orchards around the state and rearing colonies of the most common species for PPV transmission tests. Depending on the aphid species and host plant, aphid populations can be characterized as either colonizers or migrants. Colonizing aphids spend most of their lives on a single plant, and migrants travel from plant to plant.

Gildow suspects that migrant aphids are the guilty party in disease transmission, but he must still pinpoint which particular species causes disease transmission. There is a wide variety of the insects, even in a small community the size of a peach orchard. European research identified 40 aphid species in a single French orchard. In addition, researchers determined that there are roughly 3.5 million aphids in a typical planted orchard acre, and that between 50,000 to 300,000 aphids from that acre can visit a fruit tree in a one-year period.

“We also know that only 1 out of every 1,000 to 2,000 aphids is capable of transmitting plum pox virus,” Gildow says. “But if we can identify which species of aphid is infecting the fruit trees, then we can determine how to control them through spraying, weed management, or other methods.”

In collaboration with USDA’s Damsteegt, Gildow will be rearing populations of aphid species and releasing about 1,000 individuals into special containers filled with a mix of healthy and plum pox-infected plants. After the aphids migrate freely between infected and healthy plants and feed in the container for a week, the researchers will remove the aphids and grow the plants in a special isolated greenhouse for several months to see if the virus spreads. Once the species of aphids present in Pennsylvania orchards are identified and tests reveal which aphids are responsible for transmitting plum pox, scientists can offer concrete plans for control. One thing researchers know for sure: It is nearly impossible to eliminate aphid populations on fruit trees or any plant. Insecticide applications will not kill every aphid, and some migratory aphid species have developed resistance to most of the insecticides used to control them.

“Even if we eradicate the disease this time, it will reappear again somewhere in the United States,” Gildow says. “We will need every bit of information from this battle to deal with another outbreak.”

In the next few years, Gildow and USDA researchers will study how aphids carry the plum pox virus on a cellular level. He hopes to discover what genetic trait allows the virus to be carried on the stylet of certain aphids, and to identify which proteins within the genetic material of the aphid aid in the transmission of the disease.

Although all of Penn State’s research efforts will provide crucial pieces of information to solve the puzzle of plum pox virus, the long-term remedy for the disease may focus on plant breeding. Halbrendt points out that plant geneticists have bred countless crop varieties that are resistant to specific plant viruses—a short list includes corn, tomatoes, peppers, and wheat. USDA plant breeder Ralph Scorza has already developed a transgenic resistant plum variety that is undergoing field trials in European orchards. “It will take a long time to breed resistant fruit,” Halbrendt says. “Growers use many different peach and nectarine varieties in orchards, and each variety will have to be bred for resistance. But genetic resistance is the long-term solution.”

The chance to battle a new threat to a valuable crop can be a scientific challenge and offer professional exhilaration for those charged with dealing with it. However, for the state and University scientists focused on plum pox virus, personal feelings have been pushed aside. “If we weren’t so close to the growers, and this were not such a devastating disease, this would be a really interesting disease to work on,” Welliver says. “But it has already cost our growers too much. I just want it to go away.”

Faculty and extension staff referenced in this article are Herb Cole, professor of agricultural science; Tom Garretson, extension agent in Adams County; Frederick Gildow, professor of plant pathology; John Halbrendt, associate professor of plant pathology; Jayson Harper, associate professor of agricultural economics; Lynn Kime, extension agent in Adams County; William Kleiner, Adams County extension director; and James Travis, professor of plant pathology. Research is funded by the Pennsylvania Department of Agriculture, USDA, and Penn State’s Extension Program Priority Initiative.