Robert H. Walker

Influence of weed management and cropping systems on sicklepod (Cassia obtusifolia) seed in the soil.

Research was conducted from 1979 through 1982 in east-central Alabama to monitor changes in sickle- pod (Cassia obtusifolia L. #3 CASOB) seed numbers in the soil under various cropping and weed management systems in soybeans (Glycine max (L.) Merr.). Significant declines in sicklepod seed numbers were attained only when mechan- ical summer fallow each year prevented replenishment of seed in soil. Repeated disking favored germination and emer- gence of sicklepod seed and thus caused a reduction in seed numbers. Chemical summer fallow, which relied on post- emergence control of emerged sicklepod, did not result in a disturbed seedbed and was less effective in reducing seed numbers. Permitting subcompetitive densities of sicklepod, 0.45 and 0.90 plants/m2, to reach maturity each year re- sulted in increases in seed numbers in the soil. Increases were more dramatic in conventionally planted soybeans than in no-till culture. Sicklepod produced more pods per plant in tilled culture than in no-till. Sicklepod seed dis- tribution in the upper 30 cm of the soil was not affected by the type of primary tillage. Additional index words. Tillage systems, competition, weed seeds, thresholds, Glycine max, CASOB.

Behavior of sulfentrazone in ionic exchange resins, electrophoresis gels, and cation-saturated soils.

Abstract Sulfentrazone persistence in soil requires many crop rotational restrictions. The sorption and mobility of sulfentrazone play an important role in its soil persistence. Thus, a series of laboratory experiments were conducted to mimic the soil properties of cation and anion exchange with different intermediates. The molecular characterization and ionization shift of sulfentrazone from a neutral molecule to an anion were determined using a three-dimensional graphing technique and titration curve, respectively. Sorption and mobility of 2.6 × 10−5 M 14C-sulfentrazone were evaluated using a soil solution technique with ion exchange resins and polyacrylamide gel electrophoresis, respectively. Solution pH ranged from 4.0 to 7.4. As pH increased, sulfentrazone sorption to an anion resin increased and its sorption to a cation resin decreased. Percent sulfentrazone in solution was pH-dependent and ranged between 0 to 18% and 54 to 88% for the anion and cation resins, respectively. Mobility of sulfentrazone on a 20% polyacryalmide gel resulted in Rf values of +0.02 and +0.39 for pH of 4.0 and 7.4, respectively. A double peak for sulfentrazone was detected in the polyacrylamide gel when the pH (6.0 and 6.8) was near the reported pKa of 6.56. There was no clear interaction for the sorption of sulfentrazone at 1.0 mg kg−1 to Congaree loamy sand or Decatur silty clay loam saturated with either calcium or potassium. Sulfentrazone behavior with the polyacrylamide electrophoresis gels and ion resins indicate the potential for this herbicide to occur as a polar or Zwitter ion. Sulfentrazone was adsorbed by potassium, calcium, and sodium saturated resins and subsequently desorbed using variable pH solutions. The level of sulfentrazone adsorption will vary among soil types and the amount of desorption into solution may be soil cation-dependent. Nomenclature: Sulfentrazone.

A Trp574 to Leu Amino Acid Substitution in the ALS Gene of Annual Bluegrass (Poa annua) Is Associated with Resistance to ALS-Inhibiting Herbicides

Abstract Annual bluegrass is commonly controlled by acetolactate synthase (ALS)-inhibiting herbicides in managed turfgrass. An annual bluegrass population with suspected resistance to ALS-inhibiting herbicides was collected from Grand National Golf Course in Opelika, AL (GN population). Subsequent testing confirmed resistance of the GN population to foramsulfuron, trifloxysulfuron, bispyribac-sodium (bispyribac), and imazaquin when compared to a susceptible population collected locally at Auburn University (AU population). Sequencing of the ALS gene revealed a point mutation resulting in an amino acid substitution at Trp574. Cloning of the ALS gene surrounding the Trp574 region yielded two distinct ALS gene sequences: one producing Trp574 and one producing Leu574. Trp574 to Leu has been previously correlated with resistance to ALS-inhibiting herbicides. Both AU and GN gene sequences contained other similar silent and missense mutations. This research confirms resistance of annual bluegrass to ALS-inhibiting herbicides with Trp574 to Leu amino acid substitution being the most likely mode of resistance based on past literature. Nomenclature: Bispyribac; foramsulfuron; imazaquin; trifloxysulfuron; annual bluegrass, Poa annua L.

Annual bluegrass (Poa annua) populations exhibit variation in germination response to temperature, photoperiod, and fenarimol

Abstract Laboratory studies were conducted to evaluate variation in germination response of eight annual bluegrass ecotypes (‘Augusta 4’, ‘Augusta 8’, ‘Augusta 14’, ‘Augusta 17’, ‘Auburn’, ‘Birmingham’, ‘Columbia’, and ‘Purchased’) to photoperiod, temperature, and fenarimol, a fungicide–herbicide used for preemergence annual bluegrass. Seed collected from greenhouse-grown plants and stored for > 2 mo were evaluated under 18 environments (three day and night temperatures by six day and night durations). There was a significant ecotype by environment interaction affecting annual bluegrass germination. High temperature markedly restricted germination, with only the Birmingham ecotype exceeding 20% germination at day and night temperatures of 39 and 29 C, respectively. Maximum germination of all ecotypes was observed at a day and night temperature of 19 and 10 C, respectively. Maximum germination for a specific photoperiod was not consistent across ecotypes; however, all ecotypes germinated to some degree in complete darkness, which indicates that maintaining a dense turf canopy to eliminate annual bluegrass germination may not be completely effective. Ecotypes did not differ with respect to root length response to fenarimol but did vary with respect to shoot length response. Purchased and Columbia shoot growth were the most tolerant to increasing fenarimol concentrations. This information will be used to develop improved management strategies for annual bluegrass. Nomenclature: Fenarimol, α-(2-chlorophenyl)-α-(4-chlorophenyl)-5-pyrimidine-methanol; annual bluegrass, Poa annua var. annua (L.) Timm. and Poa annua var. reptans (Hauskn.) Timm. POANN.

Physiological basis for the differential tolerance of Glycine max to sulfentrazone during seed germination

Abstract Glycine max cultivars exhibit differential tolerance to soil-applied sulfentrazone. The intent of this study was to determine the physiological basis for this differential tolerance by evaluating sulfentrazone absorption and metabolism during the earliest stages of G. max development (i.e., germinating seeds, and germinal seedlings). Imbibed seeds (24 h) of the sulfentrazone-tolerant cultivar ‘Stonewall’ absorbed 37% less sulfentrazone than the sulfentrazone-sensitive cultivar ‘Asgrow 6785’. Similarly, germinal seedlings (i.e., 60 h from start of imbibition) of the sulfentrazone-tolerant cultivars Stonewall and ‘Pioneer 9593’ absorbed 22% less sulfentrazone than the sulfentrazone-sensitive cultivars Asgrow 6785 and ‘Carver’ when exposed to sulfentrazone-containing solution for either 24 or 48 h. The amount of root-absorbed 14C-sulfentrazone that was translocated into cotyledon or hypocotyl tissues did not exceed 11% of the amount absorbed and was similar for all four cultivars. Sulfentrazone metabolism by both imbibed seeds and by germinal seedlings was independent of cultivar. Increasing the sulfentrazone concentration in the seed imbibition solution and increasing the temperature resulted in greater seedling height reduction at 10 d in Asgrow 6758 than in Stonewall. Results indicate that differential absorption during the earliest stages of development is the basis for the differential response among G. max cultivars. Comparatively limited sulfentrazone absorption by Stonewall, as reflected in acceptable seedling injury, remained relatively consistent across the range of concentrations and temperatures evaluated. Nomenclature: Sulfentrazone; Glycine max (L.) Merr., soybean.

Pesticide retention by inorganic soil amendments

Abstract Pesticide retention by eight inorganic soil amendments, the majority of which are used in turf, was evaluated using a laboratory-based technique with radiolabeled pesticides. Amendments evaluated were derived from various naturally-occurring deposits of zeolites, diatomaceous earths, and fired clays and are intended to provide long-lived, stable, and uniformly sized particles that can contribute favorable water- and nutrient-retention properties to the root zone. Sand, sedge peat, and a Marvyn loamy sand soil (Ap horizon) were included for comparative purposes. Pesticides evaluated included the herbicides imazaquin and oxadiazon and the fungicide/herbicide fenarimol. Pesticide retention was evaluated with a soil solution technique. Amendments evaluated had considerable variation in cation exchange capacity (CEC), effective CEC (ECEC), surface area (SA), and field capacity with lesser variation in particle size distribution and particle density. Scanning electron microscopy revealed that surface texture was variable but frequently rough and porous. Pesticide retention was also variable but generally more than that of sand and frequently equivalent to sedge peat. Only with fenarimol and amendments that had been Ca+2-saturated could retention be correlated with any of the individual physical or chemical parameters that are generally assumed to govern pesticide adsorption, which in this case were CEC and SA. Imazaquin retention by unaltered amendments was correlated only with the products of SA and CEC, and SA and ECEC. Retention of both oxadiazon and fenarimol by unaltered amendments could not be correlated with any individual physical and chemical parameters or products thereof. Pesticide retention by these amendments is probably the cumulative sum of both true adsorption and physical entrapment. Nomenclature: Imazaquin; oxadiazon; fenarimol, a-(2-chlorophenyl)-a-(4-chlorophenyl)-5-pyrimidinemethanol.

Triazine-Resistant Annual Bluegrass (Poa annua) Populations with Ser264 Mutation Are Resistant to Amicarbazone

Abstract Amicarbazone is a photosystem II (PSII)-inhibiting herbicide in the triazolinone family, which is similar in mode of action to the triazines. Annual bluegrass is a cool-season weed and has shown resistance to some PSII-inhibiting herbicides. The objective was to evaluate triazine-resistant and -susceptible annual bluegrass populations for potential cross-resistance to amicarbazone. Two triazine-resistant (MS-01, MS-02) and triazine-susceptible (AL-01, COM-01) annual bluegrass populations were treated with amicarbazone, atrazine, and simazine at 0.26, 1.7, and 1.7 kg ai ha−1, respectively. All herbicide treatments controlled the susceptible populations greater than 94% 2 wk after treatment (WAT). No visual injury of MS-01 and MS-02 was observed at any time following herbicide treatment. Quantum yield (&PHgr;PSII) of annual bluegrass was measured 0 to 72 h after application (HAA) to determine the photochemical effects of amicarbazone compared to other PSII inhibitors. &PHgr;PSII of triazine-susceptible populations was reduced at all measurement times by all three herbicides. However, amicarbazone decreased &PHgr;PSII of susceptible populations faster and greater than atrazine and simazine at most measurement times. Amicarbazone did not reduce &PHgr;PSII of the MS-01 population. Amicarbazone significantly reduced &PHgr;PSII of the MS-02 population during several measurement timings; however, these reductions were short-lived compared to the susceptible populations and no trend in &PHgr;PSII reduction was observed. Sequencing of the psbA gene revealed a Ser to Gly substitution at amino acid position 264 known to confer resistance to triazine herbicides. These data indicate amicarbazone efficiently inhibited PSII of susceptible annual bluegrass populations; however, triazine-resistant annual bluegrass populations with Ser264 to Gly mutations are cross-resistant to amicarbazone. Nomenclature: Amicarbazone, atrazine, simazine, annual bluegrass, Poa annua L.

Using Electrolyte Leakage to Detect Soybean (Glycine max) Cultivars Sensitive to Sulfentrazone

Abstract: Laboratory studies were conducted to determine if electrolyte leakage from either leaf tissue, germinating seeds, or excised roots correlated with previously established soil-applied field response of soybean cultivars and target weeds to sulfentrazone. Sulfentrazone-induced electrolyte leakage from leaf tissue of coffee senna (sensitive), sicklepod (tolerant), and soybean cultivars ‘Asgrow 6785’ and ‘Carver’ (sensitive) and ‘Stonewall’ and ‘DPL 3606’ (tolerant) was monitored over time. Electrolyte leakage from leaf tissues, caused by 25 ppm (65 μM) sulfentrazone, agreed directly with the known response of these weeds, but response of the four soybean cultivars was equivalent. Furthermore, sulfentrazone-induced electrolyte leakage from leaf tissue of Asgrow 6785 and Stonewall was not affected by sulfentrazone concentration as high as 100 ppm (258 μM) nor by light intensity (4 and 120 μmol/m2/s photosynthetically active radiation). For germinating seeds, sulfentrazone-induced electrolyte leakage was also independent of soybean cultivar. In contrast, electrolyte leakage from excised roots of germinal soybean seedlings did concur directly with the previously established cultivar sensitivity to soil-applied sulfentrazone. Results indicate that electrolyte leakage from excised roots of soybean germinal seedlings can be used to assess cultivar sensitivity to soil-applied sulfentrazone. Nomenclature: Sulfentrazone, coffee senna, Cassia occidentalis L. #3 CASOC; sicklepod, Senna obtusifolia L. # CASOB; soybean, Glycine max (L.) Merr. ‘Asgrow 6785’, ‘Carver’, ‘Stonewall’, ‘DPL 3606’. Additional index words: Herbicide tolerance, membrane leakage, Protox, light intensity. Abbreviations: Ie, index of relative electrolyte leakage; PAR, photosynthetically active radiation; Protogen, protoporphyrinogen; Proto IX, protoporphyrin; Protox, protoporphyrinogen oxidase.

Nontuberous Sedge and Kyllinga Species' Response to Herbicides1

Annual, cylindric, and globe sedges were controlled > 90% with a single application of MSMA at 2.2 kg ai/ha in field studies. But this same treatment controlled fragrant and green kyllingas only 69 and 52%, respectively. Control was increased to 82 and 81%, respectively, with a repeat application. Other postemergence-applied (POST) herbicides evaluated included bentazon, halosulfuron, imazapic, imazaquin, and CGA-362622. Postemergence-applied herbicides were applied either once or twice, as well as alone and in combination with MSMA. In general, a sequential application of MSMA, either alone or in combination with any of the aforementioned herbicides, except bentazon, provided maximum control of the sedge and kyllinga species evaluated. Preemergence-applied (PRE) oxadiazon and S-metolachlor, controlled annual sedge ≥ 94% at 7 wk after treatment (WAT) in field studies and 96 and 70% at 9 WAT, respectively. Dithiopyr and prodiamine provided 86 to 80% control of annual sedge over the 9-wk rating period. In a hydroponic-type laboratory study, oxadiazon and S-metolachlor were more effective than atrazine, bensulide, imazaquin, oryzalin, or simazine, in reducing seedling development of annual, cylindric, and globe sedges, and green kyllinga. Nomenclature: Atrazine; bensulide; bentazon; CGA-362622 (proposed common name trifloxysulfuron), N-([(4,6-dimethoxy-2-pyrimidinyl)amino]carbonyl)-3-(2,2,2-trifluoroethoxy)-pyridin-2-sulfonamide sodium salt; dithiopyr; halosulfuron; imazapic; imazaquin; S-metolachlor; MSMA; oryzalin; oxadiazon; prodiamine; simazine; annual sedge, Cyperus compressus L. #3 CYPCP; cylindric sedge, Cyperus retrorsus Chapm. # CYPRT; fragrant kyllinga, Kyllinga odorata Vahl. [Cyperus sesquiflorus (Torr.) Mattf. and Kuekenth.](no code); globe sedge, Cyperus globulosus Aubl. # CYPGL; green kyllinga, Kyllinga brevifolia Rottb. # KYLBR. Additional index words: CYPCP, CYPGL, CYPRT, KYLBR, turfgrass weed control. Abbreviations: POST, postemergence; PRE, preemergence; WAIT, weeks after initial treatment; WAT, weeks after treatment.

Response of Two Annual Bluegrass Varieties to Preemergence and Postemergence Rimsulfuron1

In greenhouse studies, flowering annual bluegrass Poa annua L. var. annua (winter annual) and P. annua L. var. reptans (short-lived perennial) were treated postemergence with rimsulfuron at 54 g ai/ha in such a manner as to permit herbicide interception only by the foliage, interception only by the soil, and interception by both foliage and soil, as would occur in a typical application. The foliar-only, soil-only, and soil + foliar applications provided 57, 73, and 84% control, respectively, as averaged over all other experimental variables. Greater efficacy of the soil-only and soil + foliar applications indicated the importance of root absorption for annual bluegrass control with rimsulfuron. Adjuvant addition was beneficial to postemergence activity only at 18 g/ha, which is below the registered rate. Adjuvant-based control improvement at this rate was inconsistent and generally independent of the adjuvant type. Adding Renex-30 to 18 g/ha of rimsulfuron did increase control on an average from 16 to 51%. Postemergence efficacy was also independent of the annual bluegrass variety and growth media (field soil vs. a sand–peat mixture). Preemergence activity of rimsulfuron against germinating annual bluegrass seeds was slightly greater in the sand–peat mixture than in the soil and with var. annua than with var. reptans, and on uncovered seed compared with 0.5-cm-deep growth media covered seeds. Nomenclature: Rimsulfuron; annual bluegrass, Poa annua #3 POANN. Additional index words: Adjuvant study, herbicide absorption by plants, herbicide placement, surfactant study, turfgrass weeds. Abbreviations: WAT, weeks after treatment.