Orchard Spraying
When it comes to using herbicides for weed control in orchards, sometimes the good old standby is not the best bet, according to Ken Sapsford, Pesticide Specialist with the Ministry of Agriculture.
Sapsford gave weed management strategies to growers at the BC Tree Fruit Horticulture Symposium.
“Herbicide resistance is a real possibility for BC fruit growers,” says Sapsford, who owns a 2.5 acre vineyard in Kaleden. “Resistance occurs when a weed survives a normally lethal dose.” That’s different from tolerance, which describes a weed that has always been able to survive an herbicide. Worldwide there are 36 weeds that have developed resistance to glyphosate for example, Sapsford says.
Sapsford explained that while there are 27 different modes of action registered worldwide (ways that an herbicide controls susceptible plants), only 13 of those are approved for tree fruits in Canada, and they’ve been around for a while. “1983 was the last year a new mode of action was identified,” he says. “If you are thinking there is something new coming down the pipeline to help us with resistance, it’s not happening.
“We need to make sure that the products we have are going to work for us into the future, because if resistance develops, we’ve got nothing.”
Sapsford reviewed basic good practices for herbicide use. Make sure you read the label to get the best affect for the product, he advises. “Many of the soil residual products need water to move them into the soil, so irrigate or wait for a rain event,” Sapsford says. “A lot need direct contact with the soil, so clear away leaves or pruning litter from the ground, before you apply.”
With foliar applications, watch out for wet leaves. “With a heavy dew, you’ve just increased your water volume rate thousands of times,” says Sapsford. “The product will actually wash off the leaves.”
Be sure the weeds are actively growing. “A plant that is dormant or under stress wont take in the product that is meant to kill it,” he says.
Target annual plants when they are small. “A lot of products that we are introducing into orchards are designed to work on weeds in the two to four leaf stages,” Sapsford explains. “Those products do not control a large plant.”
Sprayer calibration is also important. “When you are applying soil residual products, too high a rate can damage some crops, as soil residuals are there for a longer term.” says Sapsford. “The old saying, ‘if a little is good more is better’, does not apply.”
Monitor the results after application. Check back a couple of times says Sapsford and look for plants that were supposed to die and did not. “If certain plants did not die, they may be developing a resistance and need to be hand controlled,” he says. “Don’t let the seeds of those plants escape, as the seeds will grow resistant plants as well.”
Look for IPM alternatives and consider cover crops, says Sapsford. “We know that competing crops help to control weeds,” he says. “Resistance to glyphosate has developed when it was the only product used to control weeds on bare ground.”
Plants will adapt and find a way to survive under any amount of pressure, Sapsford points out. “We have found that Canada Fleabane has developed resistance to glyphosate in Ontario and Kochia shows resistance to it in Western Canada.” Kochia is here in the Okanagan Valley Sapsford adds. “I’ve seen it in my own vineyard.”
If you use only one product, after a certain length of time a weed species will develop resistance. A way to extend that time is to use two different products with different modes of action in alternating years. Rotating herbicides is an effective way to counter resistance, but you can extend the time before resistance develops even longer, by tank mixing herbicides. Tank mixing two products with different modes of action on the same weed will effectively double the time it takes for a plant to develop resistance,” Sapsford explains.
Sapsford reviewed products from the list of BC approved modes, and described how they would complement glyphosate in a tank mix application. He says it is important to pay attention to what weeds are on your property. “Glyphosate does such a good job of controlling most weeds, that we started to ignore what was in our orchard,” says Sapsford. “Pick a product to tank mix that will target the weeds that you have.”
If you were seeking to control broadleaf weeds and crab grass, “Authority” from Group 14 would be a good tank mix. Sapsford says. If Redroot Pigweed and Nightshade are a problem, “Dual Magnum” from Group 16 might be a good choice.
“It is not recommended to tank mix “Ignite” from Group 10, or “Gramoxone” from Group 22 with glyphosate,” Sapsford cautions. Gramoxone is no longer available to purchase and product in the current formulation must be applied by Dec 31 2018, he adds. A new formulation may be available in 2019.
“Helping to delay weeds from developing resistance to the herbicides requires a four pronged approach,” says Sapsford. “Use good integrated pest management, tank-mix herbicides with multiple modes of action, use the proper rate and monitor your orchard after application.