Growers dealing with weed pressures can often encounter varying levels of herbicide resistance. Senior Specialist in Pest Management at the Almond Board of California, Drew Wolter explained that resistance is growing as new chemistries for management are slow to develop. Of the 30 confirmed occurrences of resistance in California, 13 of those weeds can often be found in almonds. As growers work to manage weed issues in the orchards, there are some important factors to consider in relation to herbicide resistance.
“I think folks can confuse or mix up something between herbicide treatment failures and actual development of resistance,” Wolter noted. “Resistance is one that was originally susceptible and over time you’ve lost that ability to control that weed. Tolerance was the inherent ability of that species to never be controlled by that herbicide or that mode of action.”
Growers and PCAs are encouraged to consider four critical points when assessing potential resistance in an orchard. How often has the same mode of action been used in the orchard suffering from weed pressures? Are the same management practices that have been effective in the past no longer addressing those weeds? Healthy weeds that are mixed in with uncontrolled weeds of the same species can also be an indicator of herbicide resistance. Finally, a patch of an uncontrolled weed type beginning to spread could mean resistance is developing. Diligent scouting is a crucial factor in determining if resistance is present in an orchard or not.
“Look for those signs that I covered where you’re scouting for development of patches or a weed that you used to kill that you haven’t been able to kill this year. Do your scouting,” Wolter explained. “The second piece to that is proper identification. Properly identifying a weed will let you know whether or not you are using the right mechanical control or chemical control; the right control method for that species.”
Listen to Wolter’s interview below.
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