COVER STORY
Herbicide Resistance
in Turf, Nursery, and Landscape Ornamentals – Can it Happen on Properties you Maintain? By Jeffrey Derr, Ph.D. and Adam Nichols, Virginia Tech This article was originally published in Virginia Turfgrass Journal, July/August 2020.
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erbicide-resistant weeds in turfgrass, ornamental, and nursery crop commodities have been an increasing issue over the past 10 years. It has been a while since we have seen a new herbicide mode of action so we are primarily relying on herbicides developed 20, 30, 40 or more years ago. Until new herbicide modes of action are developed, the issue of herbicide resistance will increase.
Definition of resistance
How resistance develops
First, some definitions here. When I say “Herbicide Resistance,” I am referring to weed species that used to be controlled by a given herbicide, but now are no longer controlled. This is different from weed species that were always difficult to control — we refer to these as troublesome or tolerant weed species. Examples of troublesome weed species would include species like kyllinga, wild garlic, wild violets, Virginia buttonweed, bindweed, poison ivy, and mugwort (wild chrysanthemum). These troublesome weeds can be controlled, but it may require higher application rates or repeat applications. For herbicide-resistant weeds, increasing the application generally has no effect, as the biotype often can tolerate many times the highest use rate.
It is thought that in a population of a given weed species, there may be a few individuals that, through a genetic mutation developed resistance to an herbicide. The mutation may have been present before that herbicide was ever used on the property. Current thinking is that herbicide application does not cause the genetic mutation that confers resistance, but that the mutations occur as a separate, random process. Often the herbicide provided a high level of control for the susceptible biotypes of that weed species. Repeated applications of that herbicide quickly control the susceptible biotypes, allowing the resistant biotype to spread through uninhibited seed production. If that herbicide is reapplied every year or so for say eight or nine years, the resistant population
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TENNESSEE TURFGRASS FEBRUARY/MARCH 2021 Email TTA at: info@ttaonline.org
