The HoneySweet plum is sweet and flavorful and highly resistant to the plum pox virus.

The HoneySweet plum is sweet and flavorful and highly resistant to the plum pox virus.

The U.S. Environmental Protection Agency has announced its conditional registration of a plum pox virus resistance gene contained in a new plum variety called HoneySweet.

Dr. Ralph Scorza, research horticulturist at the U.S. Department of Agriculture’s research station in Kearneysville, West Virginia, led a team of scientists who developed the plum-pox–resistant plum. He believes it has potential both as a source of resistant germ plasm in plum breeding and as a high-quality commercial variety for growers in areas that might be prone to plum pox virus outbreaks.

Plum pox has destroyed well over 100 million stone fruit trees throughout Europe, according to the USDA. In 1999, the virus was found in Pennsylvania, and by 2006, it was reported in New York, Michigan, and Ontario, Canada. Thousands of acres of fruit trees have been destroyed in an effort to eradicate the disease from North America. Symptoms of plum pox infection include leaf and fruit yellowing, fruit deformation, and premature fruit drop. A tree suffering from plum pox can go into serious decline, especially if the tree also becomes infected with other viruses, according to the USDA.

To develop the resistant plum variety, Scorza and his colleagues started with germ plasm from the Bluebyrd plum. They extracted embryos from the seeds and incorporated a coat-protein gene from the plum pox virus into the embryonic tissue, which was then grown into new plum trees. The gene enables the plum to recognize the plum pox virus as an invader and activate its defenses.

This is the first bioengineered temperate fruit variety to be approved by the U.S. Environmental Protection Agency, although other bioengineered crops are in widespread use, Scorza said. A papaya that is resistant to the papaya ring spot virus has been in commercial production for many years. Its virus resistance is very similar to that of the HoneySweet plum. Other bioengineered tree fruits are in various stages of development.


Scorza had to submit data packages for the plum to three regulatory agencies:  the USDA’s Animal Plant Health Inspection Service, which deregulated it; the U.S. Food and Drug Administration; and the U.S. ­Environmental Protection Agency.

According to the EPA, the coat-protein gene in the plum is a plant-incorporated protectant active ingredient, which had to be registered under Section 3 of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). Although the gene is nontoxic and not thought likely to cause adverse effects on the environment, the EPA considers it a pesticide. The agency granted a conditional registration of the gene in May, following a month-long public comment period. Most of those who commented favored registration.

Breeding resource

Scorza said the new plum could serve as germ plasm for the breeding of other virus-resistant varieties, as the resistance gene is a dominant trait and it is easy to select for. It would allow breeders to very rapidly deploy the resistance into varieties that are specifically suited to other growing environments.

“It also could be used as a variety for people interested in growing it where they are dealing with plum pox,” he said. “If plum pox should become a major problem in the United States, this could be useful.”

The variety was tested for a number of years in Europe, where the virus is still a problem, and provided an incredibly high level of resistance along with very good fruit quality, Scorza reports. Biosafety studies were conducted in both Europe and the United States.

“We’ve really shown that this technology is very effective and it’s very safe,” he said. “We have exotic insects and diseases coming long. We have climate change. We’re really looking for ways that are going to allow us to respond quickly so we can be proactive in protecting U.S. agriculture. Plum pox virus right now is pretty much under control in the United States, but we’re ready with a defense for it, should it really blow up.”

Scorza said the EPA registration is conditional for one year because the ARS needs to submit data from an independent lab to validate its methods for determining that the gene is in the plum.

The new plum is not registered in Europe, which has its own ­regulatory system.