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Three backyard fruit trees test positive for PPV

The discovery of three plum-pox-infected fruit trees in a homeowner’s
backyard in Grimsby last summer has been an unfortunate, yet minor,
setback, in what has been an eight-year, $180-million battle to
eradicate the disease in Niagara.

An example of what plum pox symptoms can look like on a tree leaf. Photo courtesy of USDA ARS

The discovery of three plum-pox-infected fruit trees in a homeowner’s backyard in Grimsby last summer has been an unfortunate, yet minor, setback, in what has been an eight-year, $180-million battle to eradicate the disease in Niagara.

It takes three years of negative test results to declare an area free of the disease that saps sugar from the fruit, but does no damage to the tree. When the current government funding for plum pox eradication ends in 2011, all of Niagara below the Niagara Escarpment will still be designated as the only infected area in Canada. 

There had been hope the Grimsby area could be plum pox free by then to show that a disease-free foothold had been made in the extreme west end of Canada’s largest tender fruit growing area.
Otherwise, there’s been positive news to report from leaf testing last summer. 

“There were very few positives samples in sub Area A (west of St. Catharines) and, in (commercial) orchards in Grimsby, there were no positive samples,” said Violet Galvin, project manager for the PPV
zeradication program in Ontario.

The most heavily infected areas are Area B – located east of Highway 55 in Niagara-on-the-Lake – and Area C – a “hot spot” area inside Area B where PPV was first detected in 1999. 

In a press release distributed in early November, the Canadian Food Inspection Agency (CFIA) that oversees the leaf testing and tree removal program said the three trees that were found in the backyard have a strain of PPV (PPV-Rec) that’s only been recently discovered and known to occur only in Europe.

“We can’t do a trace back and say definitively where it came from,” Galvin said.

The unique PPV strain was found on single apricot, peach, and plum trees planted by a previous homeowner. The trees were grafted off the same plum tree rootstock.

She said it’s similar to the PPV-W strain that was found on a single tree in a backyard in Winona six years ago. At that time, leaf sampling around the infected tree was stepped up to see if it had spread. Nothing was found. The infected tree was removed and the strain was never detected again in Canada.

“We sent inspectors into the woods to sample anything that could be a host and didn’t find anything,” Galvin recalled. She believes in this case, the newly detected PPV strain is isolated to the three trees and likely spread on the rootstock.

More samples, fewer positives
Last summer, there was a significant drop in the number of positive samples even though there was a 35 per cent year-over-year increase in the number of samples collected. Of the 750,000 leaf samples tested, only 131 were positive. This compared to 261 positives in 2007 from some 200,000 fewer samples.

“That many more samples (tested) and half the number of positive trees,” Galvin said is a good indication the program is working. “We’re doing more targeted sampling based on research on where the virus is on the tree certain times in the summer and up our sampling every year. The labs are getting very good at handling the (increased) number of samples.”

The CFIA policy of wholesale removal of trees in a 20-metre “buffer block” around an infected tree, rather than taking a surgical
approach and removing only infected trees, has been a contentious issue with growers.

“We’re concerned there’s no scientific support for a buffer block or any distance and to this point we’ve asked CFIA to reconsider,” said Adrian Huisman, secretary-manager of the Ontario Tender Fruit Producers’ Marketing Board (OTFPMB).

Currently, if a positive sample is found three consecutive years in the same block of trees, or if infection is over one per cent, growers would lose an entire block (same variety planted at the same time in the same area) of trees.

For some time, a number of large growers had asked for a go-slow approach to large-scale removals that could cause them to lose market share and the CFIA took a more compassionate view – a “special consideration policy” – to tree removal.

A year ago last fall, tree removal was capped at 10 per cent of total acreage of bearing trees and the same policy was extended this past fall.

“We know that’s too hard a hit for any grower. Some would have lost as much as 30 per cent and many more 15 per cent (acreage),” said Galvin.

It’s applicable to only a handful of growers and many are in the “hot spot” area of contamination in the McNab area of Niagara-on-the-Lake, said Len Troup, chairman of the OTFPMB. “It’s (CFIA) looking at the viability of each farm. They were really getting clobbered.” 

He added it’s an excellent way to show some compassion and allow these growers to buy time so they can remain in business.

Increased winter testing
Currently, a third season of winter testing of dormant wood has been intensified in order to identify and remove infected trees in high-risk blocks before they leaf out and flying aphids can possibly spread the disease. Testing was scheduled to begin in early December and some 100,000 samples are expected to be tested compared to 40,000 samples last winter, and just a handful of samples when winter testing began three years ago.

Winter testing was a recommendation of an international panel of experts on the disease and how it spreads.

“We’re going to find them sooner or later anyway,” Troup said about the stepped-up, off-season testing program that augments leaf testing.

He believes funding for the PPV eradication program will be extended beyond 2011 if it’s shown the disease is being contained in a smaller and smaller area and less money is needed.

(In 2004, the federal government approved an $85-million program to eradicate PPV  – $65-million for testing/ $20-million for tree removal and replanting – and, in 2007, bumped that up an additional $45.6 million, largely to pay for grower compensation. An initial $50 million for a three-year period from 2001 to 2003 was spent largely on leaf testing.)   

“Were moving along very nicely and I think there’s an acceptance this deal will be done. No one’s going to quit on it. It’s too far down the road,” said Troup, adding the United States has corralled the problem to a few identifiable trees and has vowed to eradicate PPV no matter what.

Canada would be shooting itself in the foot if it had the only PPV-infected area in North America, he said. “If anyone thinks Ontario can sit back and back off, they can forget it in a North American environment.” ❦