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Government staff in Ontario and British Columbia have told orchardists to be on the lookout for herbicide-resistant weeds, which could compete for nutrients with mature trees and make it harder to establish new plantings.

“Glyphosate-resistant Canada fleabane has been confirmed in 28 counties throughout the province,” wrote Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) weed specialist Kristen Obeid in a July 2017 information sheet for growers. “It is finding its way from fields of corn, soybeans and wheat into orchards and vineyards at an amazing rate.”

Similarly, at the BC Tree Fruit Horticultural Symposium in February 2018, British Columbia Ministry of Agriculture pesticide specialist Ken Sapsford told growers herbicide resistance is “a real possibility.”

“Be on the lookout for weeds that do not die after application when others of the same species are being controlled,” he warns growers whenever he discusses weed management.

The vigilance is warranted, said David Machial, chair of the BC Fruit Growers’ Association’s crop protection committee and an orchardist in Oliver. His family has a 20-acre orchard, including approximately 12 acres of apples and the rest a mix of stone fruits, including cherries, peaches and nectarines.

“Speaking from my own personal experience, I’m definitely seeing some of the Roundup-resistant weeds,” he says. “I’d say in another five years for sure, everyone will be seeing it and looking at other options.”

Two or three species in particular are showing signs of resistance, he said, including clover.

“Some of the clover, the Roundup will harm it a little bit, but it really doesn’t control it, so I actually go around and pull out some of the clover by hand,” he said. “I know it’s just a matter of time before it becomes better established. Natural selection will kick in … and they’ll become the dominant ones eventually.”

That’s the sort of scenario Darren Robinson, an associate professor with the University of Guelph, knows would be a disaster. Robinson has spent 17 years researching herbicide resistance in vegetables at the school’s Ridgetown campus.

He has worked with tomato growers who squared off against triazine-resistant lambsquarters and had to get a new herbicide registered for use against the weed, before it was brought back under control. The process is complicated and time-consuming, meaning growers would rather work with what they have before seeking a new registration.

“You couldn’t see the tomatoes, it was literally wall-to-wall lambsquarters,” he said. “In tree fruit crops, you will — depending on the crop — get the same issues.”

He considers the pressure from herbicide-resistant weeds greatest during the initial years of an orchard’s establishment.

“Once you have a tree that’s, say, 8 feet tall, you’re not going to worry about it that much,” he said. “During that establishment phase, root growth is absolutely essential. You can’t damage the roots, so you’re relying on chemical control or hand-weeding. And man, (labor) is so expensive.”

Machial, no stranger to hand-pulling clover, said herbicide-resistant weeds pose a threat throughout an orchard’s life.

Weeds compete with trees for nutrients, especially in high-density systems where the trees have shallower root systems than in the old full-size orchard blocks. The competing weeds can also grow into the orchard canopy as well as infrastructure such as irrigation lines, especially the new microsprinklers growers are adopting for more efficient water use.

“Weeds can obstruct the water, and then you end up with lots of rain shadow,” he said. “You’re not getting the full irrigation coverage that you need.”

Despite the practical challenges, growers have been largely left to their own devices.

Obeid and Sapsford both downplayed the incidence of herbicide resistance in orchards and vineyards. Obeid, for her part, declined to be interviewed for this story. Despite last year’s bulletin, OMAFRA communications staff said evidence of herbicide resistance among Ontario weeds is only anecdotal.

“She wouldn’t necessarily want to talk about the ‘presence’ since it has not been confirmed,” communications staff said.

Sapsford spoke more confidently but said a thorough study had yet to be done.

“There is very little confirmed herbicide resistance in British Columbia, but that may be because no one has been looking and documenting it,” he said, noting that investigating herbicide resistance is not part of his job as the province’s pesticide specialist.

Machial noted the range of issues growers face means that weed management frequently rates as a lower priority. Roundup has worked so well for so long that it’s the least of their worries compared to new insects and diseases that can devour, disfigure and destroy a crop.

“Yes, the Roundup-resistant weeds are moving into our orchards,” said Machial. “As growers, we should start using other chemical groups and start rotating to manage it. That’s what we do with our other pests, so logically it makes sense to do that with our weeds. But when you have these other threats that are potentially more damaging, or more threatening, you focus your resources on those pests.” •

—by Peter Mitham