Two populations of Palmer amaranth from New Mexico have been confirmed to be resistant to glyphosate. In the present study, the molecular basis of resistance and the mode of inheritance of resistance in those populations were investigated. Quantitative real-time polymerase chain reaction analysis indicated up to an eightfold increase in genomic EPSPS copy number in glyphosate resistant plants compared with susceptible plants. The relative genomic EPSPS copy number of resistant plants was positively correlated with the relative EPSPS cDNA expression levels. Eight hours after treatment with glyphosate, the shikimate accumulation levels in resistant plants were negatively correlated with the genomic EPSPS copy numbers. Multiple sequencing of the EPSPS cDNA of resistant plants did not reveal any glyphosate resistance-conferring mutations. The evaluation of F₁, reciprocal F₁, and F₂ Palmer amaranth families indicated that resistance to glyphosate does not follow a single-gene segregation pattern. Results suggest that the EPSPS amplification is the primary molecular basis of resistance in glyphosate resistant populations of Palmer amaranth from New Mexico.

Nomenclature: Glyphosate, Palmer amaranth, Amaranthus palmeri S. Watts.

Exposure of soybean to dicamba can result in leaf malformation and sometimes yield loss, but it is unclear how yield components are affected by exposure to low quantities of this herbicide. The objectives were to characterize soybean injury and quantify changes in seed yield and yield components of soybean plants exposed to dicamba, and determine if seed yield loss can be estimated from visual injury ratings. Nine dicamba rates (0, 0.06, 0.23, 0.57, 1.1, 2.3, 4.5, 9.1, and 22.7 g ae ha⁻¹) were applied at three growth stages (V2 – two trifoliates, V5-five trifoliates, or R2-full flowering soybean) to Beck's brand ‘342NRR’ soybean planted near Lafayette, IN, in 2009 and 2010 and near Fowler, IN, in 2009. Visually estimated soybean injury of 20% at the V2, V5, or R2 timing was 0.676 to 0.937 g ha⁻¹ dicamba at 14 d after treatment (DAT) and 0.359 to 1.37 g ha⁻¹ dicamba at 28 DAT. Seed yield was reduced by 5% from 0.042 to 0.528 g ha⁻¹ dicamba and a 10% reduction was caused by 0.169 to 1.1 g ha⁻¹ dicamba. The number of seeds m⁻², pods m⁻², reproductive nodes m⁻², and nodes m⁻² were the most sensitive yield components. Path analysis indicated that dicamba reduced seeds m⁻², pods m⁻², reproductive nodes m⁻², and nodes m⁻² which were the main causes of seed yield loss from dicamba exposure. The correlation of seed yield loss and visual soybean injury was significant (P < 0.0001) for both the V2 treatment timing (R²  =  0.92) and the V5 and R2 treatment timings (R²  =  0.91). Early-season injury rating of 8% at the V2 treatment and 2% at the V5 or R2 treatments caused 10% or more yield loss.

Nomenclature: Dicamba, soybean, Glycine max (L.) Merr.

Mechanisms of herbicide resistance were studied in a quizalofop–ethyl-resistant barnyardgrass biotype. Acetyl-coenzyme A carboxylase (ACCase) sensitivity to quizalofop-p-ethyl was measured by high-performance liquid chromatography and the trend in ACCase gene expression over time was determined using real-time polymerase chain reaction. The results showed that an insensitive ACCase was present in Geqiushan resistant plants (R), with a resistance index of 106. The basal ACCase activities in Geqiushan R and Geqiushan susceptible plants (S) were similar, at 1.20 and 1.17 ng malonyl-CoA min⁻¹ µg⁻¹ extract protein, respectively. Basal ACCase gene expression in Geqiushan R was similar to that in Geqiushan S. The relative expression of ACCase gene decreased after spraying quizalofop–ethyl at 60 g ai ha⁻¹ in Geqiushan S, whereas it was almost not changed in Geqiushan R. From these results we concluded that plastid ACCase sensitivity change might be responsible for the resistance and gene overexpression does not play a role in this resistance.

Nomenclature: Quizalofop-P-ethyl, barnyardgrass, Echinochloa crus-galli (L). Beauv. ECHCG.

Glyphosate-resistant (GR) weeds, including giant ragweed, are among the most challenging weeds for growers to control in cotton. A field study was conducted in 2011 and 2012 to determine the competitiveness of giant ragweed with densities of 0, 0.1, 0.2, 0.4, 0.8, or 1.6 plants m⁻¹ of row. Early in the growing season, giant ragweed competition with densities of at least 0.8 plants m⁻¹ row reduced cotton height compared with the weed-free control. Based on node above white flower (NAWF) and node above cracked boll (NACB) data, a delay in cotton maturity was observed for treatments with giant ragweed present at a density of 1.6 m⁻¹ of cotton row for NAWF and 0.8 m⁻¹ or 1.6 m⁻¹ of row for NACB. Lint yield losses of 50% were estimated for cotton with rows growing along side of giant ragweed at a density of 0.26 plants m⁻¹ row. Cotton in rows located 140 cm away from giant ragweed required an estimated 1.85 plants m⁻¹ row to reduce yield by 50%. These data suggest that giant ragweed sphere of influence was at least 1 m wide. Cotton fiber quality was not affected by giant ragweed at any density. Giant ragweed is a highly competitive weed in cotton, even at low densities, and efforts should be implemented to control giant ragweed early in the season to prevent cotton yield loss.

Nomenclature: Cotton, Gossypium hirsutum L., giant ragweed, Ambrosia trifida L., glyphosate resistant, GR.

Musk thistle is an invasive weed that is widely distributed throughout much of North America, including grasslands in temperate climates of the midwest USA. A series of laboratory and greenhouse experiments were conducted to determine the effect of various environmental factors on germination of musk thistle seeds. In temperature-fluctuation experiments, seed germination was greater than 65% in both alternating (30/20 C) and constant (20 or 25 C) temperature regimes with an 8-h day but less (33%) in warmer regimes (35/20 C). Germination of musk thistle seeds was 37% in alternating temperature regimes of 30/20 C in total darkness, but less than 67% in pots in the greenhouse. Differences of 10 and 15 C between day and night temperatures resulted in 91 and 75% maximum germination of musk thistle, respectively. Increasingly dryer soils reduced germination of musk thistle seeds from 35% (−0.03 MPa) to 0% (−1.2 MPa), whereas saline soils (> 80 mM) reduced maximum germination to less than 10%. Musk thistle seeds collected from populations in a bare-ground area had 96% germination, which was greater than that of seeds collected from populations growing in a perennial grass pasture (71%). A residence time (i.e., period that seeds remained on the parent plant) of 9 to 12 wk after capitulum maturity resulted in seeds germinating more quickly than those dispersed earlier. Overall, reduced light levels, cool and fluctuating temperatures, and amount of time seeds remained in residence are some of the most important factors that contribute to germination of musk thistle seeds. Information on germination dynamics of musk thistle seeds provides an understanding of the interactions that affect this process and underscores the importance of timely management strategies in temperate grasslands.

Nomenclature: Musk thistle, Carduus nutans L. CRUNU.

Dispersal mechanisms of the alien plant species buffalobur during its invasion of cold desert areas in Xinjiang, northwestern China, were investigated. Seeds and fruits were readily moved by water in irrigation canals in the transition zone between natural desert and a farmed oasis. Maximum flotation time in moving canal water was ∼ 4 h for seeds and > 48 h for fruits, and water moved fruits 279 m in 10 min. Also, 100% of the seeds remained viable during 8 wk of flooding in the laboratory. Mean dispersal distance was 3.4 m by wind-driven rolling of detached plants and 0.5 m by ants. Retention time for 50% of fruits on wool of live sheep was ∼ 4 h. Seeds and fruits that fall into the canals (which are without irrigation water from mid-October to April) are cold-stratified during winter, and then during canal cleaning in spring soil and germinable seeds are deposited along the sides of the canals. The disturbed soil is a highly favorable site for plants to grow. The local spread of buffalobur away from the sides of canals is facilitated by sheep, wind, and ants. We conclude that water in the irrigation canals is the primary dispersal agent for seeds of this invasive species and that the best way to control its spread is to prevent plants growing beside the canals from setting seed.

Nomenclature: Buffalobur, Solanum rostratum Dunal SOLRO.

The environment in which a plant grows (maternal environment) can affect seed viability, germination, and dormancy. We assessed the effects of maternal environment on wild oat seed viability, germination, dormancy, and pathogen infection by collecting and analyzing wild oat seed from above and below a barley canopy at three field sites in Montana. The viability of wild oat seed collected below a crop canopy was consistently less than it was for seed from the overstory but varied among sites and years. Reductions in viability because of relative canopy position ranged from 10% to 30%. Effects of position relative to crop canopy on weed seed germination/dormancy rates varied by site and suggest that the direction and magnitude of the effects of maternal environment on dormancy depend on environmental conditions. These effects may be driven by crop competition or by changes in seed pathogen pressure or both. Seven species each of fungi and bacteria were isolated from wild oat seeds. The only fungi causing reductions in seed viability (15%) was isolated from understory seeds, and several bacteria from both overstory and understory sources reduced seed germination. Results suggest that, in addition to the known weed-suppressive effects of using taller or earlier emerging varieties of crops, such crops can reduce weed spread through effects on weed seed demography because weeds growing beneath the crop canopy produce a reduced amount of viable seed that is less likely to germinate in the following year.

Nomenclature: Wild oat, Avena fatua L. AVEFA, barley, Hordeum vulgare L.

Postdispersal processes play an important role in the regulation of weed population dynamics. Experiments were conducted at two locations in Arkansas to understand postdispersal loss of five arable weed species important to this region—barnyardgrass, johnsongrass, pitted morningglory, Palmer amaranth, and red rice—between seed dispersal in autumn and the production of fresh seeds the subsequent autumn. Total seed loss through predation, decay, germination (fatal or successful), and loss in viability was estimated, and the influences of residue level and seed burial depth (near ground vs. 5 cm deep) were also examined. On average, the active (i.e., viable) seedbank proportion in spring (5 mo after dispersal) ranged from 8 to 11% (barnyardgrass), 10 to 11% (johnsongrass), 20 to 23% (pitted morningglory), 4 to 6% (Palmer amaranth), and 5 to 10% (red rice) across the two locations. At 1 yr after dispersal, 0.7 to 1.5% of barnyardgrass, 7 to 8% of johnsongrass, 5 to 9% of pitted morningglory, about 1.5% of Palmer amaranth, and 0.2 to 0.7% of red rice were part of the active seedbank for the two locations. There was no evidence to suggest that establishing a vegetation cover (such as a rye cover crop) after harvest of the main crop could accelerate seed predation. Burial depth did not influence seed decay, but most (45 [pitted morningglory] to 99% [Palmer amaranth]) of the seeds retrieved from the predator feeding stations were found buried in the soil substrate, and thus, not available for most predator species. This suggests that practices that allow weed seeds to lie on the soil surface (such as no-till planting in autumn) are highly valuable in encouraging seed predation. The high levels of seed loss observed in this study indicate that seedbank management should be a vital component of integrated weed management strategies.

Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv., johnsongrass, Sorghum halepense (L.) Pers., Palmer amaranth, Amaranthus palmeri S. Wats., pitted morningglory, Ipomoea lacunosa L., red rice, Oryza sativa L., cereal rye, Secale cereale L.

Periodic surveys were conducted to seek potential indigenous fungal agents for development as mycoherbicides against horse purslane, a major weed of agricultural fields in India. Pathogenic fungal species were isolated and identified from naturally infected horse purslane. The biocontrol potential of these pathogens for horse purslane was evaluated by studying their host range and virulence under growth chamber and greenhouse conditions. Three candidates, Alternaria alternata, Fusarium oxysporum, and Phoma herbarum, were identified as potential candidates for biological control of horse purslane. Preliminary host-range tests and pathogenicity studies, conducted using 45 crop and weed plants belonging to 18 families, demonstrated that P. herbarum provided effective weed control and was safe to most of the plant species tested. Further mycoherbicidal application of P. herbarum as plant spray under field condition caused mortality of horse purslane 60 d after application of the inoculums. Phoma herbarum is a good mycoherbicide candidate against horse purslane.

Nomenclature: Alternaria alternata (Fr.) Keissler, Fusarium oxysporum Schltdl., Phoma herbarum Westendorp, horse purslane, Trianthema portulacastrum L. TRTPO.

The development of dicamba-tolerant and other auxin-tolerant crops will enable the use of these effective herbicides in soybean and cotton at application timings such as at planting or over-the-top that are not currently possible. This research examined the effect of various factors on detection of postapplication amounts of dicamba in the air under field conditions by coupling a sample collection system with advanced chemical analysis of those samples. The quantity of dimethylamine salt of dicamba that was detected within 48 hr after application was two times greater (P < 0.05) than the quantity of diglycoamine salt formulation based on field studies in 2009. Regardless of application timing, the amount of detected dicamba was greatest during the 0 to 12 hr time period after application. However, the total detected after 48 hr was less for evening applications (5 micrograms [µg]) compared with midday (17 µg) or morning (14 µg) applications based on 2010 field trials. Average ambient air temperature (and other weather variables) correlated with higher detection levels of dicamba in the air in the field.

Nomenclature: Dicamba, soybean, Glycine max (L.) Merr.

Synthetic auxin herbicides are widely utilized in golf course settings for selective broadleaf weed control. Aminocyclopyrachlor (AMCP) is a newly registered pyrimidine carboxylic acid with similar chemical mode-of-action and structure to triclopyr (TRIC) and clopyralid (CLPY). Off-target injury on terrestrial plants has been documented following exposure to turfgrass clippings previously treated with TRIC and CLPY. Management practices on golf courses can distribute turfgrass clippings into water bodies; however, research has not evaluated the bioavailability of synthetic auxin residues from turfgrass clippings to aquatic and riparian plants within these environments. A bioassay study was conducted to determine the response of alligatorweed and parrotfeather to tall fescue clippings previously treated with synthetic auxin herbicides. Previously treated AMCP and TRIC CLPY clippings were placed into growth containers mimicking a lentic system containing both alligatorweed and parrotfeather. Results indicated all herbicide treated clippings induced significant growth responses to alligatorweed and parrotfeather growth compared to a nontreated mulch and nontreated control. Alligatorweed control was greater from AMCP clippings treated 14, 7, 3, and 1 DBCC (49, 60, 90, and 80%, respectively) than comparative TRIC CLPY clippings (33, 25, 37, and 64%, respectively) at 10 weeks after treatment (WAT). Parrotfeather control was greater from AMCP clippings (57 to 87%) than TRIC CLPY clippings (9 to 63%) collected from all days before clipping collection (DBCC) timings when evaluated 6 WAT. At 10 WAT, greater parrotfeather control and shoot reduction was observed from AMCP than TRIC CLPY clippings when treated 14, 7, and 3 DBCC. Based on these data, synthetic auxin residues can become bioavailable to aquatic and riparian plants within aqueous environments.

Nomenclature: Aminocyclopyrachlor, clopyralid, triclopyr, alligatorweed, Alternanthera philoxeroides (Mart.) Griseb., parrotfeather, Myriophyllum aquaticum (Vell.) Verdc., tall fescue, Lolium arundinaceum (Schreb.) S.J. Darbyshire.