By Vicky Boyd
Although herbicide resistance isn’t developing as quickly in permanent crops as it is in row crops, it is nonetheless something to be concerned about and to manage.
Key to slow resistance is rotating herbicide modes of action, using non-chemical control such as disking or flaming, and crop rotation where possible.
That was the message presented to attendees of the Unified Wine and Grape Symposium by John Roncoroni, a University of California Cooperative Extension farm adviser in Napa County specializing in weed control.
He had just returned from the Pan American Weed Resistance Conference in Miami and heard horror stories from counterparts in Australia, where some weeds are resistant to seven different herbicide families.
Once a weed becomes resistant to one class of chemicals, it can quickly turn into a super weed and grow resistant to multiple modes of action, Roncoroni says.
”Australia had to go back to non-herbicide weed control,” he says.
Complicating weed management is the inexpensive price of glyphosate. When faced with other weed management choices, glyphosate frequently wins out because of its low cost per acre, Roncoroni says.
Of the 21 herbicides that are registered for grapes, glyphosate accounts for 62 percent of herbicide use in Napa County vineyards, he says. Chateau follows with 16 percent of the acres followed by 13 percent for Goal.
Glyphosate costs about $29 per acre, including application, Roncoroni says, citing UC cost of production studies.
Chateau followed by one application of glyphosate runs $74 per acre, whereas Rely can run$92 per acre.
That compares with $39 per pass per acre for cultivation and $54 per pass per acre for flaming. A grower typically would need to cultivate or flame twice during the season.
Fish Friendly Farming practices encourage farmers to use practices that reduce the runoff of sediment into streams. Depending on the vineyard slope and soil type, cultivation can increase sedimentation, so many growers avoid the practice.
That’s one reason why glyphosate is used so extensively, Roncoroni says. It is considered a safe herbicide under the Fish Friendly Farming recommendations.
”Diversity is the key to sustainability,” he says. “Glyphosate is a once-in-a-generation pest control tool we cannot afford to lose. But at the rate we’re going, we’re going to lose it.
”We need to look at some other forms of weed control. Spend a little money now or we are going to lose it.”
Four weeds have been confirmed resistant to herbicides in California vineyards and orchards.
They are rigid ryegrass, Italian ryegrass, hairy fleabane and horseweed—also known as marestail. All of these are resistant to glyphosate. In addition, hairy fleabane appears to be resistant to paraquat, Roncoroni says.
They join common groundsel, which was confirmed as herbicide resistant in asparagus in 1981.
Ronconi says these weeds are truly resistant to glyphosate.
Other weeds are classified as tolerant and may have never really been controlled by a specific chemical. Among those are filaree, malva or cheeseweed and tall annual willow herb. Glyphosate wasn't ever really effective on them, he says.