Kristen E. McNaughton

Mutations in ALS confer herbicide resistance in redroot pigweed (Amaranthus retroflexus ) and Powell amaranth (Amaranthus powellii )

Abstract A number of redroot pigweed and Powell amaranth populations from various locations in Ontario, Canada, have distinct patterns of resistance to the acetolactate synthase–inhibiting herbicides imazethapyr and thifensulfuron. This suggested the presence of diverse ALS gene mutations among these populations. Seven polymerase chain reaction primer pairs were used to amplify the gene to obtain full sequence information and to determine the identity of resistance-conferring mutations. There was a high degree of similarity in the ALS gene of the two species with only five nucleotides and one amino acid differing. A total of four herbicide resistance-conferring mutations were identified in the two species. The Ala122Thr, Ala205Val, and Trp574Leu amino acid substitutions were found in redroot pigweed whereas Ala122Thr, Trp574Leu, and Ser653Thr were detected in Powell amaranth. The pattern of resistance known to be conferred by the mutations concurred with the resistance level observed at the whole plant level. Distinct mutations being found in geographically separated populations suggest that selection for resistance occurred simultaneously in different locations. It reinforces the fact that resistance to ALS inhibitors is easily selected and that growers need to take this into account when formulating weed management strategies. Nomenclature: Imazethapyr; thifensulfuron; Powell amaranth, Amaranthus powellii S. Wats. AMAPO; redroot pigweed, Amaranthus retroflexus L. AMARE.

Multiple resistance to imazethapyr and atrazine in Powell amaranth (Amaranthus powellii)

Abstract Multiple-herbicide resistance represents an added weed management challenge to growers as it can considerably reduce their options for weed control. The widespread nature of triazine resistance in Ontario coupled with the more recent appearance of resistance to ALS inhibitors in Amaranthus species warranted documenting biotypes with multiple resistance. A collection of Powell amaranth and redroot pigweed biotypes that had previously been characterized for resistance to ALS inhibitors was therefore screened with atrazine. Dose–response analysis with atrazine and imazethapyr was also conducted. High-level resistance to imazethapyr and atrazine was determined in a Powell amaranth biotype from Perth County, Ontario. This biotype had a > 1,860-fold and 109-fold resistance to atrazine and imazethapyr, respectively. Sequence analysis was conducted for the psbA and ALS genes that code for the target sites of the triazines and imidazolinones, respectively. A mutation in the psbA gene was identified that coded for an amino acid substitution of glycine for serine at residue 264 of the D1 protein. This mutation is the most likely cause for triazine resistance in this biotype. Similarly, a nucleotide substitution was identified that codes for threonine in place of serine at position 652 of the ALS protein. This mutation in the ALS gene has only been observed previously in laboratory-selected mutants of arabidopsis and tobacco and is known to endow resistance to imidazolinones in plants. It is concluded that multiple resistance in this Powell amaranth biotype is due to the presence of altered target sites for triazine and imidazolinone herbicides. Nomenclature: Atrazine; imazethapyr; Powell amaranth, Amaranthus powellii S. Wats. AMAPO; redroot pigweed, Amaranthus retroflexus L. AMARE; Arabidopsis thaliana (L.) Heynh. Arabidopsis; tobacco, Nicotiana tabacum L.

Saflufenacil Carryover Injury Varies among Rotational Crops

Abstract Trials were established in 2007, 2008, and 2009 in Ontario, Canada, to determine the effect of soil residues of saflufenacil on growth, yield, and quality of eight rotational crops planted 1 yr after application. In the year of establishment, saflufenacil was applied PRE to field corn at rates of 75, 100, and 200 g ai ha−1. Cabbage, carrot, cucumber, onion, pea, pepper, potato, and sugar beet were planted 1 yr later, maintained weed-free, and plant dry weight, yield, and quality measures of interest to processors for each crop were determined. Reductions in dry weight and yield of all grades of cucumber were determined at both the 100 and 200 g ha−1 rates of saflufenacil. Plant dry weight, bulb number, and size and yield of onion were also reduced by saflufenacil at 100 and 200 g ha−1. Sugar beet plant dry weight and yield, but not sucrose content, were decreased by saflufenacil at 100 and 200 g ha−1. Cabbage plant dry weight, head size, and yield; carrot root weight and yield; and pepper dry weight, fruit number and size, and yield were only reduced in those treatments in which twice the field corn rate had been applied to simulate the effect of spray overlap in the previous year. Pea and potato were not negatively impacted by applications of saflufenacil in the year prior to planting. It is recommended that cabbage, carrot, cucumber, onion, pepper, and sugar beet not be planted the year after saflufenacil application at rates up to 200 g ha−1. Pea and potato can be safely planted the year following application of saflufenacil up to rates of 200 g ha−1. Nomenclature: saflufenacil; cabbage, Brassica oleracea var. italica Plenck.; carrot, Daucus carota L.; corn, Zea mays L.; cucumber, Cucumis sativus L.; onion, Allium cepa L.; pea, Pisum sativum L., pepper, Capsicum sativa L.; potato, Solanum tuberosum L.; sugar beet, Beta vulgaris L.

Effect of application timing of glyphosate and saflufenacil as desiccants in dry edible bean (Phaseolus vulgaris L.)

McNaughton, K. E., Blackshaw, R. E., Waddell, K. A., Gulden, R. H., Sikkema, P. H. and Gillard, C. L. 2015. Effect of application timing of glyphosate and saflufenacil as desiccants in dry edible bean (Phaseolus vulgaris L.). Can. J. Plant Sci. 95: 369-375. Early application of desiccants in dry edible bean may cause yield reductions and unacceptable herbicide residue levels, resulting in rejection of exported shipments. The effect of application timing of two registered desiccants, glyphosate and saflufenacil, was examined in 12 field trials conducted over a 4-yr period (2009-2012) at Exeter, Ontario, Carman, Manitoba, and Lethbridge, Alberta. Desiccants were applied alone and in combination at five crop maturation stages. When glyphosate or saflufenacil alone, or in combination, was applied at 100% crop maturity, herbicide residue levels were acceptable (less than 2.0 and 0.01 ppm for glyphosate and saflufenacil, respectively) and there was no reduction in yield or hundred seed weight. Glyphosate residues remained below 2.0 ppm when the desiccant was applied alone or with saflufenacil at 75% crop maturity, but crop yield decreased by 16% compared with the untreated control when glyphosate and saflufenacil were combined. Residue levels were unacceptable when glyphosate was applied at 0, 25, and 50% maturity; generally the earlier glyphosate was applied, the greater the residue concentration in the seeds at harvest. Although no application timing resulted in saflufenacil residues above 0.01 ppm, crop yield was reduced when the desiccant was applied at 0, 25, 50, and 75% crop maturity. This information will provide dry bean processors with the necessary information to design guidelines concerning the application timing of glyphosate and saflufenacil so that bean yield and quality remain unaffected and seed residues remain below accepted levels.

Effect of five desiccants applied alone and in combination with glyphosate in dry edible bean (Phaseolus vulgaris L.)

McNaughton, K. E., Blackshaw, R. E., Waddell, K. A., Gulden, R. H., Sikkema, P. H. and Gillard, C. L. 2015. Effect of five desiccants applied alone and in combination with glyphosate in dry edible bean (Phaseolus vulgaris L.). Can. J. Plant Sci. 95: 1235-1242. Application of dry bean desiccants just prior to crop maturity is common practice by Canadian producers. As dry beans are grown for human consumption it is critical that producers pick desiccants that do not affect crop yield, seed quality, or result in desiccant seed residue levels above accepted levels. In this study the efficacy of glyphosate, diquat, glufosinate, carfentrazone, flumioxazin, and saflufenacil as desiccants was examined for navy, cranberry, pinto, and great northern dry bean. Seed herbicide residues were also tested for each of the dry bean classes tested. Navy, cranberry, pinto, and great northern dry bean yields were not impacted by use of the desiccants diquat, carfentrazone, flumioxazin, or saflufenacil when applied at labelled rates and application timings. Additionally, herbicide residues in seed following application remained lower than maximum residue limits (MRL) established by primary Canadian dry bean export partners. Generally, dry bean colour, irrespective of class, was not altered by desiccant use; diquat and flumioxazin caused minor increases in the degree of red and yellow seed pigmentation for cranberry bean only. Although colour differences were noted using a Chroma meter the differences were slight and would not likely be of economic importance. Application of glyphosate did not affect crop yield, and seed residue levels were below MRLs for navy, pinto, and great northern bean. However, seed glyphosate residue levels were above the MRL for cranberry bean when glyphosate was applied alone or tankmixed with carfentrazone, flumioxazin, or saflufenacil. Seed residue levels were also above listed MRLs for some export countries when glufosinate was applied to navy, cranberry, and pinto bean, although crop yield and seed quality remained unaffected. These findings suggest that growers and contractors should avoid using glufosinate as a dry bean desiccant at least for some markets and that care should be taken when selecting glyphosate as a desiccant, especially for cranberry bean. Across all market classes desiccation progress of bean leaf, stem, and pod tissue was slowest when glyphosate and carfentrazone were used.

Weed control in soybean using pyroxasulfone and sulfentrazone

Belfry, K. D., McNaughton, K. E. and Sikkema, P. H. 2015. Weed control in soybean using pyroxasulfone and sulfentrazone. Can. J. Plant Sci. 95: 1199-1204. Pyroxasulfone and sulfentrazone are new herbicides currently being evaluated for weed control in soybean [Glycine max (L.) Merr.] in Ontario, Canada. Seven experiments were conducted over a 3-yr period (2011 to 2013) at Ridgetown and Exeter, Ontario, to evaluate weed management using pyroxasulfone, sulfentrazone and their tank-mixes relative to the industry standard, imazethapyr plus metribuzin. Tank-mixing pyroxasulfone and sulfentrazone provided up to 97, 46, 60, 100 and 71% control of common lambsquarters (Chenopodium album L.), common ragweed (Ambrosia artemisiifolia L.), green foxtail [Setaria viridis (L.) Beauv.], Powell amaranth [Amaranthus powellii (S.) Wats.] and velvetleaf (Abutilon theophrasti Medic.), respectively, at 2 wk after treatment. Control with pyroxasulfone and sulfentrazone was improved when tank-mixed, relative to application of each herbicide separately. Although control was variable across weed species, no difference in control was identified between pyroxasulfone plus sulfentrazone and imazethapyr plus metribuzin. Soybean yield was up to 2.7, 2.4 and 2.9 t ha-1 for pyroxasulfone, sulfentrazone and pyroxasulfone plus sulfentrazone application, yet imazethapyr plus metribuzin provided the highest yield (3.3 t ha-1). This research demonstrates that pyroxasulfone plus sulfentrazone may be used as a valuable weed control option in soybean; however, weed community composition may limit herbicidal utility.

Response of Processing Tomato to Simulated Bromoxynil Drift Followed by In- Crop Metribuzin Application

Eight field studies were conducted over a 3-yr period from 2008 to 2010 in Ridgetown, Ontario, Canada, to determine the cumulative stress caused by simulated glyphosate spray drift followed by an in-crop application of metribuzin in processing tomato. As the simulated glyphosate spray drift rate increased so did the degree of injury to the tomatoes. At a simulated spray drift rate of 22.5 g ae ha 1 (2.5% of the recommended glyphosate field rate), a 23% decrease in red tomato yield was observed. Yield reductions increased to 88% of the control when 180 g ae ha 1 glyphosate (20% of the recommended field rate) was applied. Similarly when simulated spray drift rates were followed 3 to 5 d later with an in-crop application of metribuzin at 250 g ai ha , tomato yields decreased by 22 to 85% depending on glyphosate rate applied. A transient synergistic interaction was observed only when 22.5 g ae ha 1 glyphosate was followed by metribuzin. The synergistic response was no longer evident by the 28-d injury rating. Herbicide interactions were additive for crop injury, dry weight, fruit counts, and yield when glyphosate spray drift rates of 45, 90, or 180 g ae ha 1 were followed by metribuzin. Nomenclature: Glyphosate; metribuzin; tomato, Solanum lycopersicon L.

Soybean (Glycine max) Tolerance to Timing Applications of Pyroxasulfone, Flumioxazin, and Pyroxasulfone + Flumioxazin

Abstract Four field studies were conducted over a 3-yr period (2011 to 2013) to determine the tolerance of four soybean cultivars to pyroxasulfone (89 and 178 g ai ha−1), flumioxazin (71 and 142 g ai ha−1), and pyroxasulfone + flumioxazin (160 and 320 g ai ha−1) applied either preplant incorporated (PPI), PRE, or at the soybean cotyledon stage (COT). When pyroxasulfone + flumioxazin was applied at 160 and 320 g ai ha−1, at the cotyledon stage soybean yield was decreased by 9 and 14%, respectively. The only other treatment that decreased soybean yield was pyroxasulfone (178 g ai ha−1) applied PPI; yield was decreased by 6% despite minimal injury and dry biomass reductions observed during the season. Soybean tolerance to pyroxasulfone or flumioxazin applied alone was generally similar and injury was less than with pyroxasulfone + flumioxazin. Similarly, herbicides applied PPI and PRE were less injurious to soybean than the COT timing. Results suggest that soybean is tolerant to PPI and PRE applications of pyroxasulfone + flumioxazin but COT applications should be avoided. Nomenclature: Flumioxazin; pyroxasulfone; soybean, Glycine max (L.) Merr. Resumen Se realizaron cuatro estudios de campo durante un período de 3 años (2011 a 22013) para determinar la tolerancia de cuatro cultivares de soya a pyroxasulfone (89 y 178 g ai ha−1), flumioxazin (71 y 142 g ai ha−1), y pyroxasfulone + flumioxazin (160 y 320 g ai ha−1) aplicados ya sea incorporados en presiembra (PPI), PRE, o en el estado cotiledonal de la soya (COT). Cuando se aplicó pyroxasulfone + flumioxazin a 160 y 320 g ai ha−1 en el estado cotiledonal, el rendimiento de la soya se redujo en 9 y 14%, respectivamente. El único otro tratamiento que disminuyó el rendimiento de la soya fue pyroxasulfone (178 g ai ha−1) aplicado PPI, en el cual el rendimiento se redujo 6% a pesar de que el daño y reducciones de biomasa seca observados fueron mínimos durante la temporada de crecimiento. La tolerancia de la soya a pyroxasulfone o flumioxazin aplicados solos fue generalmente similar y el daño fue menor que con pyroxasulfone + flumioxazin. Similarmente, los herbicidas aplicados PPI y PRE fueron menos dañinos a la soya que al aplicarse COT. Los resultados sugieren que la soya es tolerante a aplicaciones PPI y PRE de pyroxasulfon + flumioxazin, pero las aplicaciones COT deberían ser evitadas.

Control of Glyphosate-Resistant Giant Ragweed in Winter Wheat

Four field experiments were conducted over a 2-yr period (2012 and 2013) in winter wheat to evaluate POST herbicides for the control of glyphosate-resistant (GR) giant ragweed. POST herbicides were evaluated for winter wheat injury and GR giant ragweed control, population density, and aboveground biomass. The herbicides used in this study provided 54 to 90% and 51 to 97% control of GR giant ragweed at 4 and 8 wk after treatment (WAT), respectively. At 8 WAT, auxinic herbicide treatments or herbicide tank mix/premix treatments that contained auxinics provided 78 to 97% control of GR giant ragweed. Reductions in GR giant ragweed population density and aboveground biomass were 62 to 100% and 83 to 100%, respectively, and generally reflected the level of control. The results of this research indicate that Ontario, Canada, corn and soybean growers should continue to incorporate winter wheat into their crop rotation as one component of an integrated weed management (IWM) strategy for the control of GR giant ragweed. Nomenclature: Glyphosate; giant ragweed, Ambrosia trifida L.; winter wheat, Triticum aestivum L. Cuatro experimentos de campo fueron realizados durante un período de dos años (2012 y 2013) en trigo de invierno para evaluar herbicidas POST para el control de Ambrosia trifida resistente a glyphosate (GR). Se evaluó el efecto de los herbicidas POST sobre el daño en el trigo de invierno y el control, densidad de población y la biomasa aérea de A. trifida GR. Los herbicidas usados en este estudio brindaron 54 a 90% y 51 a 97% de control de A. trifida GR a 4 y 8 semanas después del tratamiento (WAT), respectivamente. A 8 WAT, los tratamientos con herbicidas tipo auxina o mezclas/pre-mezclas en tanque que contenían herbicidas tipo auxina brindaron 78 a 97% de control de A. trifida GR. Las reducciones en la densidad de la población y la biomasa aérea de A. trifida GR fueron 62 a 100% y 83 a 100%, respectivamente, y generalmente reflejaron el nivel de control. Los resultados de esta investigación indican que los productores de maíz y soja de Ontario, Canada, deberían continuar incorporando el trigo de invierno en sus rotaciones de cultivos como un componente de una estrategia de manejo integrado de malezas (IWM) para el control de A. trifida GR.

Herbicide Tolerance of Lima Bean (Phaseolus lunatus) in Ontario1

Lima bean cultivar ‘Improved Kingston’ was evaluated for sensitivity to herbicide treatments in a field experiment, conducted from 1999 to 2001 in Ontario. Crop was evaluated for visual injury at 7, 14, and 28 d after emergence following preemergence (PRE) applications of metolachlor (1,600 and 3,200 g ai/ha) and imazethapyr (75 and 150 g ai/ha). Crop visual injury to postemergence (POST) applications of imazamox + fomesafen (25 + 200 g ai/ha and 50 + 400 g ai/ha) and quizalopfop-P (72 and 144 g ai/ha) was evaluated at 7, 14, and 28 d after treatment. Plant height and crop yield were also assessed. The imazamox + fomesafen mixture caused significant visual injury and tended to decrease lima bean height and yield. Despite some initial injury observed in the metolachlor, imazethapyr, and quizalofop-P treatments, yield was not significantly decreased. Because of their margin of crop safety, metolachlor applied PRE at 1,600 g/ha, imazethapyr applied PRE at 75 g/ha, and quizalofop-P applied POST at 72 g/ha have excellent potential as weed management tools in Ontario lima bean production. Nomenclature: Fomesafen; imazamox; imazethapyr; metolachlor; quizalopfop-P; lima bean, Phaseolus lunatus L. Additional index words: Herbicide injury, sensitivity. Abbreviations: DAE, days after emergence; DAT, days after treatment; OM, organic matter; POST, postemergence; PPI, preplant incorporated; PRE, preemergence; UAN, urea ammonium nitrate.