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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), flazasulfuron (Katana), rimsulfuron, metsulfuron (MSM, Manor, others), and rimsulfuron + metsulfuron (Negate), chemicals used to selectively control cool-season grasses in bermudagrass and certain other warm-season turfgrasses.

We have verified a biotype of annual bluegrass from a golf course that is resistant to the commonly used sulfonylurea herbicides in bermudagrass. The golf course had been using Monument yearly to control annual bluegrass but after years of use had noted reduced control. In this case, one would have to look at preemergence applications of chemicals like Barricade, Specticle or simazine, or use postemergence treatments of glyphosate or Kerb (pronamide).

Recently, annual bluegrass resistant to early postemergence applications of Specticle (indaziflam) was detected at several locations in the southern U.S. Of extra concern in that research was that one of those annual bluegrass biotypes that tolerated early post-emergence applications of Specticle also tolerated postemergence applications of flumioxazin, foramsulfuron, glyphosate, metribuzin, pronamide, and simazine. So that biotype could tolerate six different herbicide modes of action. How can one control such a biotype?

Recently, annual bluegrass biotypes have been identified in Georgia that exhibit differential susceptibility to protoporphyrinogen oxidase (PPO) inhibitors, which includes Ronstar (oxadiazon) and flumioxazin (SureGuard, others). Whether it is due to genetic variability in this species or some other reason, clearly annual bluegrass is prone to developing herbicide resistance.

Other weeds besides annual bluegrass have developed resistance to certain herbicides (Table 1). Goosegrass has developed resistance to Ronstar in Virginia and to the dinitroaniline herbicides in Tennessee. Lawn burweed in New Zealand and buckhorn plantain in Indiana has developed resistance to the synthetic auxins, so the problem is not limited to grassy weeds. Compressed sedge in Alabama has developed resistance to key sedge herbicides, including halosulfuron (Sedgehammer, Prosedge). Large crabgrass and smooth crabgrass have developed resistance to the postemergence grass herbicides, which includes Acclaim Extra (fenoxaprop), as well as sethoxydim (Segment) and fluazifop (Fusilade, Ornamec).

A concern I have is if crabgrass species in Virginia develop resistance to quinclorac (sold by itself under the name Drive, and as a component of combination products such as Q4 Plus, among others.) Quinclorac is the primary herbicide used for postemergence crabgrass control in turf situations and is one of the few postemergence options in bermudagrass. One of the studies we have been conducting at the research station is evaluating alternative herbicides to quinclorac for postemergence crabgrass control in bermudagrass. There are more alternatives to quinclorac for postemergence crabgrass control in cool-season turf.

Table 1 lists resistant weeds reported from general turf areas to the weedscience.org website. It is not a complete list as some cases of resistance in turfgrass have not been reported to this website but it does an indication of the problem. These listings in Table 1 do not mean all populations of those weeds in the states listed are resistant, just that resistance has been documented in at least one location in that state. As these weeds flower, mowers can move seed from one site to another. Seed can also spread through wind, soil or sod, moving the resistant biotype to new locations.

Table 1. Herbicide resistant weeds reported from turf to the International Herbicide-Resistant Database, www.weedscience.org)

MOAs = mode of action. The letters and numbers listed by the MOAs are from the Weed Science Society of America’s classification system (numbers) and the Herbicide-Resistance Action Committee’s classification system (letters).

Herbicide resistance in the nursery and landscape maintenance industries

Despite substantial reliance on herbicides for weed management, herbicide-resistant weeds have generally been perceived to be less of an issue in the nursery crop and landscape maintenance industries compared with larger-acreage agronomic cropping systems. Annual bluegrass, common groundsel (Senecio vulgaris), horseweed (Conyza canadensis), and fringed (northern) willowherb (Epilobium ciliatum) have been reported to be herbicide resistant in nursery production in at least one country (Table 2). A majority of the reports are weeds developing resistance to the Photosystem II (PS II) inhibitors, such as simazine (Princep). Although simazine is commonly used in field nursery production, it is not used in maintenance of landscape ornamentals.

One weed that has developed resistance to glyphosate in Virginia and other states is horseweed, often called marestail. Resistance developed primarily due to the extensive use of glyphosate in agronomic crop production. Horseweed seed can blow a considerable distance in the wind, so it can spread from crop fields to nursery production areas, landscape beds, and non-crop areas.

Generally combinations of preemergence herbicides are used in nursery production and in maintenance of landscape ornamentals to broaden the spectrum of weed control. The herbicides being combined usually differ in their mode of action so applying such combinations helps to reduce the potential for resistance development. A common combination as an example would be combining Gallery (isoxaben), more effective on annual broadleaf weeds, with Barricade (prodiamine), more effective on annual grasses.

These two herbicides differ in their mode of action. Besides combinations of sprayable herbicides, there are quite a few granular formulations that contain two active ingredients. An example would be Snapshot, a combination of isoxaben and trifluralin.

If herbicide-resistant weeds are present in nursery production areas, these biotypes could be spread to landscape beds through the planting of infested trees or shrubs. However, we do not have good information on the spread of herbicide- resistant weed populations from nurseries to landscape beds.

Table 2. Herbicide-resistant weeds reported for the nursery industry throughout the world as reported by Heap 2019 (International Survey of herbicide resistant weeds, http://www.weedscience.org/).

Preventing herbicide resistance

What can we do to prevent the development of herbicide-resistant weeds? One should rotate herbicide modes of action or look at tank mixes of herbicides with different modes of action. One does not need to do this every year, but maybe every three or four years. For example, one could alternate use of a dinitroaniline herbicide like prodiamine with Specticle (indaziflam) or oxadiazon in bermudagrass turf on a golf course. Simazine could be alternated with flumioxazin at a tree nursery. Halosulfuron could be alternated with sulfentrazone (Dismiss) for yellow nutsedge control in woody landscape beds and in turf areas. Alternating between FreeHand and Snapshot would be an option in ornamental beds. Utilize both preemergence and postemergence herbicide where possible as these 2 groups generally have different modes of action. To use this strategy, one needs to learn how each available herbicide controls weed (their mode of action). Use non-chemical means of weed control in your control program. Hand weed plants that survive an herbicide application, thus preventing seed production. Clean equipment when going to other locations to prevent spread of weed seed. By following these suggestions, there is less potential for herbicide resistance to appear on properties you maintain.

Jeffrey Derr, Professor of Weed Science, and Adam Nichols, Turfgrass Research Manager, are located at Virginia Tech’s Hampton Roads Agricultural Research and Extension Center in Virginia Beach. Trade names are listed only for information purposes as examples, and do not imply discrimination of products not mentioned.