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These transgenic plums contain a gene that makes them resistant to plum pox virus. Photo by Scott Bauer, USDA Agricultural Research Service

These transgenic plums contain a gene that makes them resistant to plum pox virus. Photo by Scott Bauer, USDA Agricultural Research Service

A genetically modified plum variety that’s resistant to plum pox virus could help protect the U.S. soft fruit industry against a major outbreak of the virus.

Dr. Ralph Scorza, plant breeder at the U.S. Department of Agriculture’s research lab at Kearneysville, West Virginia, developed the resistant variety, which is called Honeysweet, and is working towards gaining regulatory approval for it. Currently, the only genetically modified tree fruit planted anywhere in the world is papaya.

Scorza submitted paperwork to the U.S. Department of Agriculture’s Animal Plant Health Inspection service in March—16 years after beginning the project. After review, the USDA will take public comment. He must also gain approval from the Food and Drug Administration and the U.S. Environmental Protection Agency.

When Scorza began developing the resistant plum in 1990, he knew plum pox virus was an invasive, potentially devastating pathogen, though it was not found in the United States at that time.

His project involved transferring a small, noninfectious piece of the virus into plum and regenerating that tissue into a plant. It involved basic research to show why the new plants were resistant, so the method might be used again for transforming other stone fruits, and field trials in which plants were put to the test with inoculations of the virus. He was unable to do field inoculations in the United States, because the virus was not found there, so the tests were done in Spain, Poland, and Romania, where the virus is commonly found. It required a lot of paperwork to allow a genetically modified plant to be tested, and every single fruit had to be harvested.

That was followed by hybridization to transfer the resistance into seedlings that might be potential new varieties. The transgenic clone now known as Honeysweet proved to be highly resistant to the plum pox virus.

Though it was far more complex and costly to develop the variety through genetic transformation rather than traditional breeding, scientists have not been able to achieve that level of resistance through conventional means, Scorza noted.

In the meantime, an outbreak of plum pox virus in Pennsylvania in 1999 demonstrated how devastating the virus could be. About 1,600 acres of peach, nectarine, and apricot orchards in Pennsylvania were torn out in an effort to eradicate the virus, which can affect most Prunus species.


Scorza believes his transgenic plum is important for two reasons. It will demonstrate how the process of regulatory approval might work for other transgenic fruits, but it also provides germplasm that can be used to protect the industry from the virus in future.

“In the past, we’ve had diseases that have come through, like chestnut blight, Dutch elm disease, sudden oak death, or citrus canker, and we really haven’t been prepared for them in the sense of having resistant germplasm,” he said. “What we’ve had to do is go back and work on an emergency basis.”

The new resistant variety will provide an important source of resistant germplasm that can be used by breeders and studied by other scientists so that if a major outbreak of the virus does occur in the United States the stone fruit industry will be protected and the risk of it spreading into native Prunus trees will be reduced.

“That, to me, is the key,” he said. “It’s not so much that tomorrow people are going to be seeing Honeysweet plums in the supermarket. That’s not what we’re looking at. This is an important source of high-level resistance.”

Scorza said the technology has broad application. It should set the stage for developing plum-pox-resistant Japanese plums, peaches, apricots, and cherries. In addition, the genetic modification technologies he employed might be used to achieve other improvements of tree fruits.