Read the entire February 2016 issue of GCM »

From the February 2016 issue of GCM magazine:

Annual sedge resistance to halosulfuron and other ALS inhibitors in turfgrass

Confirmation of resistant sedges adds to the growing problem of herbicide-resistant weeds in turf.

Sedges are troublesome weeds, and several species
have been reported to be resistant to the herbicides known
as ALS inhibitors. This paper is the first report of a sedge
in turfgrass that has shown resistance to ALS inhibitors.
Photos by Patrick McCullough

Patrick McCullough, Ph.D.; Jialin Yu, Ph.D.; and Scott McElroy, Ph.D.

Read this story in GCM's digital edition »

The introduction of halosulfuron to the turf industry in the 1990s provided a highly efficacious post-emergence herbicide for controlling annual and perennial sedge species. This herbicide provided more consistent control than bentazon (Basagran, BASF) and offered better selectivity than MSMA (monosodium acid methanearsonate). Halosulfuron is an acetolactate synthase (ALS) inhibitor with low use rates and limited injury potential to tolerant turfgrasses. Other ALS inhibitors introduced more recently for sedge control in turfgrass are flazasulfuron (Katana, PBI-Gordon), sulfosulfuron (Certainty, Monsanto) and trifloxysulfuron (Monument, Syngenta). These herbicides are highly efficacious for sedge control but are injurious to cool-season grasses. Halosulfuron is the only ALS inhibitor labeled for sedge control in all major warm- and cool-season turfgrasses.

Resistance to ALS inhibitors

Resistance to ALS inhibitors has increased exponentially during the past decade in various cropping systems. Resistance in weeds to these herbicides is attributed to either target-site alteration or enhanced herbicide degradation (3). Of the two, target-site alteration occurs more frequently and results in greater levels of ALS resistance (2). There are currently five sedge species with ALS resistance reported, including shortleaf spike sedge [Cyperus brevifolius (Rottb.) Endl. Ex Hassk], smallflower umbrella sedge (C. difformis L.), yellow nutsedge (C. esculentus L.), rice flatsedge (C. iria L.) and fragrant flatsedge (C. odoratus L.) (1). Fragrant flatsedge, rice flatsedge and smallflower umbrella sedge are annuals that reproduce from seed, while the others are perennials that reproduce from seed and tubers.

An annual sedge (C. compressus L.) biotype with suspected resistance to ALS inhibitors was identified in a bermudagrass turf in Georgia. The turfgrass manager noted reductions in annual sedge control after using halosulfuron exclusively for more than a decade. Research was conducted to evaluate (1) the level of resistance of this biotype to halosulfuron, (2) efficacy of various herbicides for control, and (3) molecular and physiological mechanisms associated with resistance.

Evaluation of annual sedge resistance to halosulfuron and other herbicides

Greenhouse experiment 1

Annual sedge plants were collected from a Tifway bermudagrass field in Georgia on Aug. 12, 2014. These plants were uninjured from halosulfuron at a standard use rate, 1.3 ounces/acre (91 grams/hectare), applied approximately three weeks before collection. Annual sedge was also collected in Griffin, Ga., from a susceptible population. The two biotypes were grown out in a greenhouse at the University of Georgia-Griffin campus. Upon maturity, seeds were collected by hand and scattered with soil (85 sand:15 peat moss) over pots 1.5 inch (3.8 cm) in diameter and 7.87 (20 cm) inches deep. Pots were fertigated (MacroN 28-7-14 sprayable fertilizer, Lesco) biweekly and allowed to reach a height of 4 inches (~10 cm) before treatments.

Figure 1. Biomass reductions for ALS-resistant and ALS-susceptible biotypes of annual sedge at eight weeks after treatment in greenhouse experiments. The herbicides evaluated were: glufosinate (Finale 1 L, Bayer), glyphosate (Roundup Pro, Monsanto), halosulfuron-methyl (Sandea/Sedgehammer, Gowan), imazapic (Plateau, BASF), MSMA (Target 6 L, Luxembourg-Pamol), sulfentrazone (Dismiss, FMC), sulfosulfuron (Certainty, Monsanto) and trifloxysulfuron-sodium (Monument, Syngenta). All herbicides were applied with a non-ionic surfactant at 0.25% volume/volume, except glyphosate. LSD = least significant difference.

The response of the two annual sedges was evaluated from a rate titration of halosulfuron (Sandea, Gowan Co.). Treatments were applied in a rate titration ranging from 0.1 to 20.8 ounces of product per acre (4.4 to 1,120 grams ai/hectare). Nontreated checks of the two biotypes were included. A non-ionic surfactant (Chem Nut 80-20, a mixture of alkyl and alkylaryl polyoxyethylene glycol, 80%; Chem Nut Inc.) was added to the spray solution at 0.25% volume/volume. Plants were returned to the greenhouse about one hour after treatment and did not receive irrigation for 24 hours.

After eight weeks, the halosulfuron rate required to reduce dry shoot biomass 50% measured 0.14 ounce/acre (8 grams ai/hectare) for the susceptible biotypes and more than 21 ounces acre (>1,120 grams ai/hectare) for the resistant biotypes. All application rates of halosulfuron reduced biomass of the resistant biotype less than 12%. Halosulfuron rates equal to or greater than 0.67 ounce/acre (35 grams ai/hectare) reduced biomass of the susceptible biotype more than 80%. Results support the supposition that the resistant annual sedge biotype could tolerate halosulfuron levels 140 times greater than those tolerated by the susceptible biotype.

Greenhouse experiment 2

In a separate greenhouse experiment, annual sedges were treated with various herbicides, including glufosinate (Finale, Bayer), glyphosate (Roundup, Monsanto), halosulfuron (Sedgehammer, Gowan), imazapic (Plateau, BASF), mesotrione (Tenacity, Syngenta), MSMA (Target 6, Luxembourg-Pamol), sulfentrazone (Dismiss, FMC Professional Solutions), sulfosulfuron (Certainty, Monsanto) and trifloxysulfuron (Monument, Syngenta). A nontreated check was included. The resistant biotype of annual sedge was not controlled by halosulfuron, imazapic, sulfosulfuron, or trifloxysulfuron (Figure 1). Conversely, these herbicides reduced biomass for the susceptible biotype by 62% to 80%. Glufosinate, glyphosate and MSMA reduced biomass 77% to 97% for both biotypes, while sulfentrazone reduced biomass of both biotypes by ~50% relative to the nontreated control. Mesotrione caused initial visual injury on both biotypes (not shown), but biomass was only reduced 17% at eight weeks after treatment.

In this field experiment, MSMA was shown to be effective
in controlling an ALS-resistant annual sedge.

Target-site inhibition differences in the resistant biotype were compared with the susceptible biotype of annual sedge. The halosulfuron concentration required to inhibit ALS enzymes 50% (I₅₀) was more than 172 times greater in the resistant biotype than in the susceptible biotype. The DNA was also sequenced for the gene that encodes for the target-site enzyme (ALS). We identified a mutation in the resistant population that was caused by the substitution of a Ser amino acid for a Pro-197 amino acid. This mutation confers resistance to ALS inhibitors in the annual sedge biotype through a target-site alteration.

Preventing resistance development in sedge populations

The repeated use of halosulfuron or other ALS inhibitors will exacerbate the spread of resistant sedges. Superintendents need to develop integrated weed management programs that prevent the establishment and spread of ALS-resistant biotypes. Applications of pre-emergence herbicides in turfgrass will be critical for managing ALS-resistant annual sedge, especially for turfgrass species susceptible to injury from other post-emergence herbicides. Dinitroaniline herbicides used for pre-emergence control of annual grassy weeds in turfgrass generally have limited efficacy on sedges. Superintendents may need to incorporate dimethenamid (Tower, BASF), oxadiazon (Ronstar, Bayer), sulfentrazone (Dismiss, FMC Professional Solutions) or S-metolachlor (Pennant Magnum, Syngenta) in pre-emergence control programs to effectively manage annual sedge populations. These herbicides may have significant limitations for use, such as cost, turfgrass injury and restrictions on labeled areas. Selecting the appropriate pre-emergence herbicide may be critical for controlling annual sedge and minimizing the spread of resistant biotypes.

ALS-resistant annual sedge populations flourish in the field.

Sulfentrazone was less effective than MSMA, glufosinate and glyphosate for controlling ALS-resistant annual sedge in greenhouse experiments. However, field experiments conducted in the summer of 2015 on ALS-resistant annual sedge revealed good (80% to 89%) to excellent (90% to 100%) control from single applications of Dismiss at 12 ounces/acre (420 grams ai/hectare). Sulfentrazone has residual activity and may be applied for pre- or post-emergence control of annual sedge. The potential loss of organic arsenical herbicides, including MSMA, could limit the mechanisms of action available to superintendents and further increase the occurrence of ALS-resistant sedges. Bentazon (Basagran, BASF) is a Photosystem II inhibitor that offers turf managers an alternative to ALS inhibitors for annual sedge control. Many superintendents have replaced bentazon in spray programs with ALS inhibitors because of improved efficacy, turfgrass safety and the fewer applications required for controlling sedges. However, bentazon should be used alone or in tank mixtures with ALS inhibitors for managing herbicide resistance in annual sedge populations. These alternative mechanisms of action will be important for controlling ALS-resistant biotypes in turfgrass.

Recommendations

This is the first report of resistance to ALS inhibitors in annual sedge, and the first report of a herbicide-resistant sedge species from a turfgrass system. The repeated use of ALS inhibitors may select for resistant biotypes of annual sedge in turfgrass. This possibility demonstrates the importance of rotating mechanisms of action. As new weed species are confirmed with resistance in turfgrass, superintendents should be educated about the importance of herbicide rotation for sustainable management.

Acknowledgments

Data presented in this article was originally published in the journal Weed Science: Patrick E. McCullough, Jialin Yu, J. Scott McElroy, S. Chen, H. Zhang, Timothy L. Grey and Mark A. Czarnota. 2016. ALS-resistant annual sedge confirmed in turfgrass. Weed Science 64:33-41.

Literature cited

1. Heap, I. 2015. The International Survey of Herbicide-Resistant Weeds. (www.weedscience.com). Accessed May 5, 2015.
2. Shaner, D.L. 1999. Resistance to acetolactate synthase (ALS) inhibitors in the United States: history, occurrence, detection, and management. Journal of Weed Science Technology 44:405-411.
3. Tranel, P.J., and T.R. Wright. 2002. Resistance of weeds to ALS-inhibiting herbicides: What have we learned? Weed Science 50:700-712.

Patrick McCullough is an associate professor and Jialin Yu is a postdoctoral research associate in the Department of Crop and Soil Sciences at the University of Georgia, Griffin, Ga. Scott McElroy, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University, Auburn, Ala.