Despite the widespread adoption and use of herbicides, weeds still persist in agricultural and horticultural systems. Weeds can escape chemical control for numerous reasons, including: incorrect herbicide or rate selection, improper sprayer calibration, clogged nozzles or otherwise malfunctioning equipment, weed size (e.g. too large for control), herbicide applications that are made under less-than-ideal environmental conditions (e.g. too cold, too windy, too wet or too dry), and the development of herbicide resistance in weed species.
The Weed Science Society of America (WSSA) defines herbicide resistance as “the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide that is normally lethal”. There are two general mechanisms of herbicide resistance: target site resistance and non-target site resistance.
Target site resistance occurs when the enzyme that is the target of the herbicide becomes "insensitive" to the herbicide that was applied. The loss of sensitivity is usually associated with a mutation in the gene that codes for the enzyme that the herbicide adheres to in the plant. These mutations lead to physical changes in the enzyme’s shape/structure, which prevents herbicide-binding, thus reducing or eliminating herbicidal activity.
Alternately, a mechanism of resistance may not be directly related to the herbicide target site; this type of occurrence is called non-target site resistance. Non-target site mechanisms can impede herbicide uptake, translocation, and accrual or else protect the plant against the actions of the herbicide.
Weed species resistant to herbicides can be found around the world, in the United States, and in Washington production systems. To date, more than 250 plant species, worldwide, have developed resistances to one or more herbicide sites of action (http://weedscience.org). Across all crops, there are: 12 unique cases of herbicide resistance in Washington, 19 unique cases in Oregon, 11 in Idaho, 14 in Montana, 3 in Wyoming, and 30 in California. Six instances of glyphosate (WSSA Group 9) resistance have been confirmed in tree and vine crops in California and Oregon including: annual bluegrass (Poa annua), junglerice (Echinochloa colona), horseweed/marestail (Conyza canadensis), hairy fleabane (Conyza bonariensis), Italian ryegrass (Lolium perenne), and rigid ryegrass (Lolium rigidum). Resistance to both glyphosate and paraquat (WSSA Group 22) has been detected in horseweed/marestail and hairy fleabane in California. Multiple resistance to glyphosate and glufosinate (WSSA Group 10) has been detected in Italian ryegrass in Oregon. Italian ryegrass with resistance to three herbicide groups (WSSA 1, 9, and 22) and four herbicide groups (WSSA 1, 2, 9, and 22) has been reported in California.
In order to delay the evolution of resistance/manage herbicide resistant weed species, consider implementing the following practices:
- Start clean and stay clean. This is especially important if perennial species are infesting a new orchard site; take time to manage difficult to control weeds prior to planting new trees.
- Apply integrated weed management practices. Use multiple herbicide modes-of-action with overlapping weed spectrums in rotation, sequences, or mixtures.
- Use cultural practices such as cultivation, mowing, cover crops or mulch, where appropriate.
- Use the recommended herbicide rate at the proper application timing for the hardest to control weed species present in the field.
- Scout orchards after herbicide applications to ensure control has been achieved. Prevent weeds from producing seed or proliferating vegetatively to prevent a buildup of propagules that can persist across years.
- Monitor orchards and clean equipment to avoid moving seed, rhizomes, or tubers, etc., between them.