Overuse of acetolactate synthase (ALS)–inhibiting herbicides in rice has led to the evolution of halosulfuron-resistant rice flatsedge in Arkansas and Mississippi. Resistant accessions were cross-resistant to labeled field rates of ALS-inhibiting herbicides from four different families, in comparison to a susceptible (SUS) biotype. Resistance index of Arkansas and Mississippi accessions based on an R/S ratio of the lethal dose required for 50% plant mortality (LD50) to bispyribac-sodium, halosulfuron, imazamox, and penoxsulam was ≥ 21-fold. Control of Arkansas, Mississippi, and SUS accessions with labeled field rates of 2,4-D, bentazon, and propanil was ≥ 93%. An enzyme assay revealed that an R/S ratio for 50% inhibition (I50) of ALS for halosulfuron was 2,600 and 200 in Arkansas and Mississippi, respectively. Malathion studies did not reveal enhanced herbicide metabolism in resistant plants. The ALS enzyme assay and cross-resistance studies point toward altered a target site as the potential mechanism of resistance. Trp574–Leu amino acid substitution within the ALS gene was found in both Arkansas and Mississippi rice flatsedge accessions using the Illumina HiSeq platform, which corresponds to the mechanism of resistance found in many weed species. Field-rate applications of 2,4-D, bentazon, and propanil can be used to control these ALS-resistant rice flatsedge accessions.

The results suggest a target-site alteration as the mechanism of resistance in yellow nutsedge, which accounts for the cross-resistance to other ALS-inhibiting herbicide families.

Smallflower umbrella sedge is a problematic weed in direct-seeded rice in the midsouthern United States. It recently has evolved resistance to the acetolactate synthase (ALS) -inhibiting herbicide halosulfuron in Arkansas rice. Studies were conducted (1) to determine if the resistant biotype is cross resistant to other ALS-inhibiting herbicides, (2) to evaluate alternative herbicide control options, and (3) to determine the mechanism of resistance. Whole-plant bioassay revealed that halosulfuronresistant plants were not controlled by bispyribac-sodium, imazamox, and penoxsulam at the labeled field rate of each herbicide. The level of resistance to these herbicides, based on the lethal dose needed to kill 50% of plants (LD 50 ) was $ 15-fold compared to a susceptible biotype. Both biotypes were controlled .96% with bentazon and propanil and # 23% with quinclorac, thiobencarb, and 2,4-D. Hence, effective control measures exist; albeit, the number of herbicide options appear limited. Based on in vitro ALS enzyme assays, altered target site is the mechanism of resistance to halosulfuron and imazamox. Massively parallel sequencing with the use of the Illumina HiSeq detected an amino acid substitution of Pro 197 -to-His in the resistant biotype that is consistent with ALS-inhibiting herbicide resistance in other weed species. Nomenclature: Bispyribace-sodium; halosulfuron; imazamox; imazethapyr; penoxsulam; smallflower umbrella sedge, Cyperus difformis L.; rice, Oryza sativa L.

Doveweed is a problematic weed of lawns and sod production, as well as golf course roughs, fairways, and tees. End-user reports of selective POST control options are inconsistent and control is often short-lived. In addition, inconsistent control with non-selective herbicides such as glyphosate is common. The goals of this research were: (1) evaluate selective POST doveweed control options in ‘Tifway’ hybrid bermudagrass turf; (2) compare efficacy of single vs. sequential applications of selective POST herbicides; (3) quantify doveweed tolerance to glyphosate; and (4) quantify recovery of foliar applied glyphosate following treatment with a C14-glyphosate solution. A single application of sulfentrazone+metsulfuron; thiencarbazone+iodosulfuron+dicamba or 2,4-D+MCPP+dicamba+carfentrazone; or thiencarbazone+foramsulfuron+halosulfuron provided >60% control 2 weeks after initial treatment (WAIT). A second application of these treatments 3 WAIT improved control 6 WAIT. Two applications of 2,4-D+MCPP+dicamba+carfentrazone or thiencarbazone+foramsulfuron+halosulfuron provided ~80% control 6 WAIT. Doveweed was tolerant to glyphosate application up to 5.68 kgaeha-1. Absorption of 14C-glyphosate was compared between doveweed with cuticle intact, doveweed with a disturbed cuticle, and smooth crabgrass. 14C-glyphosate recovery from the leaf surface of doveweed plants with an intact cuticle was 93.6%. In comparison, 14C-glyphosate recovery from the leaf surface of doveweed plants with a disrupted cuticle and the leaf surface of crabgrass plants was 79.1 and 70.5%, respectively.

Heat stress adversely affects upland cotton (Gossypium hirsutum L.) production in the United States. Heat tolerance is difficult to determine, especially when yield and fibre quality are the parameters measured in photoperiodic, non-adapted genotypes. Our objective was to evaluate wild (mostly photoperiodic, non-adapted) genotypes for vegetative heat tolerance using chlorophyll fluorescence. Parameters were measured in three phases of heat stress assays to select candidate lines for introgression of increased heat tolerance into commercial cotton germplasm. Forty-four wild accessions were selected from 1762 wild accessions in the USDA germplasm collection using a single-leaf sample assay (phase I). In the growth chamber, 6-week-old cotton plants were subjected to heat stress at 45°C and 80% humidity for 24 h. After heat stress, the mean Φ PSII for the selected accessions was higher than that of a set of randomly chosen accessions and adapted cultivars (phase II). In the field under high temperatures (>35°C), the top nine accessions had higher average Φ PSII than adapted cultivars (phase III). Based on these results, the three-phased stepwise approach was consistent in identifying putative genetic resources for increased vegetative heat tolerance. Accessions were identified that may have potential as genetic materials for development of adapted heat-tolerant upland cotton germplasm.