MTC Turf News – Jeffrey Derr, Ph.D. and Adam Nichols, Virginia Tech
Herbicide-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
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.
How resistance develops
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 will increase until it is the dominant biotype. Over that eight or nine years, that herbicide will be less and less effective until it provides no control at all.
Examples of resistance in turf situations
There are certain weed species that appear to be prone to developing herbicide resistance, such as annual bluegrass (Poa annua). There are documented cases of annual bluegrass biotypes that have developed resistance to certain herbicides. In 2019, Ian Heap (International survey of herbicide resistant weeds, www.weedscience.org) reported that annual bluegrass ranks third among all herbicide-resistant weed species globally, with resistance to nine different herbicide sites of action. Triazine-resistant annual bluegrass has been detected in turfgrass, with simazine being the primary triazine used in turf (mainly used in bermudagrass), with specialized uses of atrazine in certain warm- season turf species. Adam Nichols and I documented triazine resistant annual bluegrass at a golf course in Virginia a number of years ago. We kept seed from that biotype and discovered that biotype was also resistant to Xonerate (amicarbazone). Simazine and amicarbazone both are photosynthetic inhibitors.
Annual bluegrass biotypes resistant to the dinitroaniline herbicides have been identified in Tennessee, North Carolina, and Georgia. The dinitroaniline herbicide class incudes prodiamine (Barricade, others), pendimethalin (Pendulum, others) , oryzalin (Surflan, others), and trifluralin (Treflan, others), key turfgrass and ornamental herbicides. Annual bluegrass resistant to glyphosate (Roundup, many others) has been identified in Tennessee and Missouri.
Biotypes of Poa annua resistant to the sulfonylurea herbicides have been documented in Tennessee, Alabama, and Virginia. The sulfonylurea herbicide class includes foramsulfuron (Revolver), trifloxysulfuron (Monument),