Dec 30, 2011

Plum pox virus meets its match

A research scientist with Agriculture and Agri-Food Canada is proving to be more than a match for the plum pox virus (PPV).

Aiming Wang received Canada’s Gold Harvest Award in 2010 for developing a transgenic plum variety resistant to the disease. Now, he’s using more conventional means to develop the same in peaches.

“I think we are the leader in the world in this area,” Wang said. “We have already identified the peach gene (the virus needs).”

In order to replicate themselves, plant viruses depend on genetic material within their hosts. To find an appropriate gene – one of about 40,000 in peach trees – Wang and his team embarked on a kind of fishing trip, using one of 11 virus genes as bait.

It’s a complex process, one that required nearly four years of effort. The next step is to mutate the peach gene so that the virus can no longer use it. That may take another two or three years, Wang estimated.

The technique doesn’t involve the use of genetic engineering, Wang said. Instead, a chemical treatment of peach shoots is used to induce mutagenesis. The same type of mutations occurs naturally through the exposure of plants to UV radiation.

“It’s difficult for people to accept genetically modified food, so we needed a way to develop a peach mutant population, but without GM.”

A screening process will be used to identify the appropriate resistant peach variant.

With this accomplished, a new resistant peach variety can be propagated vegetatively and evaluated for other traits, like yield and quality. Then plant breeders can step in, using the cultivar to develop other resistant varieties. A similar approach could be used to develop resistance in plums, nectarines, apricots and other stone fruit. Wang suspects the same gene found in peach trees is involved. Plum pox virus was identified in Europe more than a century ago. There, the disease has become a problem orchardists simply live with. In Spain, for example, peaches and other susceptible tree fruit crops are regularly established in new locations. Once the virus arrives and spreads, the orchard is abandoned.

With the introduction of resistant varieties, the establishment of permanent orchards would be possible. In Canada, the virus has been eradicated in Nova Scotia but persists in the Niagara region of Ontario. In the United States and Japan, there’s a low incidence of the virus. When it’s detected in a location, all trees are removed.

Plum pox is now in the process of being eradicated in three New York counties and one Michigan county, according to data from USDA’s Animal & Plant Health Inspection Service. It’s been eliminated in Pennsylvania, where it was first detected in 1999.

Plum pox virus is spread by aphids and through the transport of infected plant material. The fruit of infected trees becomes blemished and unmarketable. There is no cure.

Wang was born on a small farm in southeastern China, where his academic prowess allowed him to attend university. He earned his Ph.D. at the University of British Columbia.

Honey Sweet

The search for a solution to the plum pox virus has been an international effort.

Ralph Scorza, a USDA plant breeder at the Appalachian Fruit Research Station in West Virginia, led the effort that began in 1990 and culminated with the release of Honey Sweet, a genetically engineered plum variety resistant to the plum pox virus.

“We received an unconditional registration in August of 2011,” Scorza said. “It’s not being planted very widely because PPV has almost been eradicated in the U.S.”

The European researchers Scorza has been working with hope to have the variety approved in Europe.

Honey Sweet is probably best suited to the fresh market. The team is working to develop other plum varieties with resistance to the virus. This is being achieved by crossing Honey Sweet with other varieties and by using the same technique used to develop Honey Sweet.

It’s more difficult to develop resistance to the virus in peaches and other stone fruit, Scorza said.

He refers to the overall approach as “pre-emptive breeding” – planning to deal with a problem that could potentially become widespread in the future. He advocates for using a broad range of technologies, together with appropriate safeguards, in dealing with insect and disease challenges in agricultural crops.

By Jeffrey Carter, Ontario Correspondent