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Simon Scott moves plants at the Musser Fruit Research Farm's greenhouse, where about 300 trees of a hundred low-chill cultivars will be grown.

Simon Scott moves plants at the Musser Fruit Research Farm’s greenhouse, where about 300 trees of a hundred low-chill cultivars will be grown.

Richard Lehnert

Until plum pox virus (PPV) appeared in Pennsylvania peach orchards in 1999, many peach growers took a pretty casual view of viruses—living with them and accepting a certain level of damage.

Plum pox changed that attitude, according to Clemson University virologist Dr. Simon Scott. Growers became much more concerned about viruses of all sorts.

As part of the National Clean Plant Network, Scott runs the Southeastern Budwood Program for South Carolina and Georgia peach growers to help assure they plant virus-free trees.

Although plum pox is transmitted in the field by aphids, both PPV and other major virus problems of peach—prunus necrotic ring spot virus (PNRSV) and prune dwarf virus (PDV)—are primarily transmitted when infected budwood is used to propagate new trees. Scott’s work is to prevent that from happening.

“Prune dwarf virus can reduce yield by about 44 percent, and prunus necrotic ring spot virus by 5 percent,” Scott said.

“If a tree gets the two viruses together, yields will be reduced by up to 55 ­percent,” he said.

Starting about April 1 each year, Scott tests trees in mother blocks, which are located on peach farms in South Carolina and Georgia. Three Tennessee nurseries are the big producers of new peach trees for the Southeast, and they collect budwood each year from trees in about 50 mother blocks. Most of them, 38, are located at Titan Farms in Ridge Springs, South Carolina, but other blocks are located in south and mid-Georgia.

“The major location in Ridge Spring is like one-stop shopping for the nurseries,” Scott said. “They can send a budding crew and complete their work quickly. The budwood must be kept cold.” The budwood is cut from new shoots on trees when shoots are about 18 inches long, and one shoot can provide six to ten buds.

Last year, a screened greenhouse was renovated at Clemson’s Musser Fruit Tree Research Farm. It is designed to hold 200 to 300 trees of about 100 cultivars growing in pots. These trees will generate small amounts of virus-free budwood and serve as a source for larger plantings. Trees in this collection are low-chill varieties that are being transferred from Prosser, Washington, which is a major repository of virus-free wood for all kinds of fruits.

“Prosser is too cold for the low-chill varieties,” Scott said, “so they are being moved here.” Most of those were either developed in Dick Okie’s USDA peach breeding program in Byron, Georgia, or in the peach breeding program at the ­University of Florida .

Low-chill varieties are those that want to break dormancy after 300 hours or less of chilling. Growers in the North plant varieties that require more than 800 hours of chilling.

The Musser Farm greenhouse needed to have good temperature control  because of the adverse effects of summer heat on the growth of trees in pots. It had to be screened to eliminate virus-vectoring insects, especially aphids. “It won’t keep out thrips,” Scott said, “but we don’t know of any viruses which infect peaches that are thrips-borne.”

The 24- by 48-foot greenhouse renovation was funded by the National Clean Plant Network.