Cory M. Whaley

A New Mutation in Plant Als Confers Resistance to Five Classes of Als-inhibiting Herbicides

Abstract Experiments were conducted to evaluate a biotype of smooth pigweed that had survived applications of sulfonylurea (SU) and imidazolinone (IMI) herbicides in a single season. The source field had a history of repeated acetolactate synthase (ALS)-inhibiting herbicide use over several years. Whole-plant response experiments evaluated the resistant (R11) biotype and an ALS-inhibitor susceptible (S) smooth pigweed biotype to herbicides from the SU, IMI, pyrimidinylthiobenzoate (PTB), and triazolopyrimidine sulfonanilide (TP) chemical families. The R11 biotype exhibited 60- to 3,200-fold resistance to all four ALS-inhibiting herbicide chemistries compared with the S biotype. Nucleotide sequence comparison of Als genes from R11 and S biotypes revealed a single nucleotide difference that resulted in R11 having an amino acid substitution of aspartate to glutamate at position 376, as numbered relative to the protein sequence of mouseearcress. This is the first report of an amino acid substitution at this position of an Als gene isolated from a field-selected weed biotype. To verify the role of this mutation in herbicide resistance, the Als gene was cloned and expressed in Arabidopsis. Transgenic Arabidopsis expressing this Als gene exhibited resistance to SU, IMI, PTB, TP, and sulfonylaminocarbonyltriazolinone ALS-inhibiting herbicide classes. Nomenclature: Chlorimuron cloransulam imazethapyr propoxycarbazone pyrithiobac thifensulfuron smooth pigweed, Amaranthus hybridus L. AMACH mouseearcress, Arabidopsis thaliana (L.) Heynh. ARBTH.

ALS resistance in several smooth pigweed (Amaranthus hybridus) biotypes

Abstract Experiments were conducted to identify acetolactate synthase (ALS, EC 2.2.1.6 [formerly EC 4.1.3.18]) mutation sites in eight biotypes of smooth pigweed and correlate these mutations with patterns of herbicide cross-resistance. Four herbicide-resistant smooth pigweed biotypes (R5, R6, R7, R8) collected from fields in Virginia, Delaware, and Maryland, showed a similar response to postemergence applications of the ALS-inhibitors imazethapyr, pyrithiobac, chlorimuron, thifensulfuron, and cloransulam. These R biotypes ranged from 261- to 537-fold resistant to imazethapyr and 29- to 88-fold resistant to pyrithiobac. The biotypes also had reduced sensitivity to chlorimuron and thifensulfuron of 2- to 14-fold and 10- to 25-fold, respectively, relative to a susceptible smooth pigweed biotype (S). Biotypes R6, R7, and R8 had reduced sensitivity of 3- to 10-fold to cloransulam relative to the S biotype, whereas R5 had increased sensitivity. All of these biotypes were found to have a serine to asparagine substitution at amino acid position 653, as numbered relative to the protein sequence of Arabidopsis thaliana. This stands in contrast to four other imidazolinone (IMI)-resistant smooth pigweed biotypes (R1, R2, R3, R4) that were collected from fields in Somerset County, Maryland. These biotypes were found to have an alanine to threonine substitution at position 122 of the ALS enzyme and were previously characterized at the whole-plant level with high-level resistance to IMI herbicides, increased sensitivity to pyrimidinylthiobenzoate and triazolopyrimidine sulfonanilide herbicides, and low to no cross-resistance to sulfonylurea herbicides. Nomenclature: Chlorimuron; cloransulam; imazethapyr; pyrithiobac; thifensulfuron; smooth pigweed, Amaranthus hybridus L. AMACH.

Comparison of Mesotrione Combinations with Standard Weed Control Programs in Corn1

Field experiments were conducted in 2002 and 2003 to evaluate total POST weed control in corn with mixtures of mesotrione, atrazine, and the commercial mixture of nicosulfuron plus rimsulfuron plus atrazine at registered and reduced rates. Treatments were compared with nicosulfuron plus rimsulfuron plus atrazine POST, and S-metolachlor plus atrazine PRE alone and followed by (fb) nicosulfuron plus rimsulfuron plus atrazine POST. All treatments controlled common lambsquarters 8 wk after the postemergence treatments (WAPT). Common ragweed control with POST mesotrione plus nicosulfuron plus rimsulfuron plus atrazine combinations was greater than 89%. Mesotrione plus the registered rate of nicosulfuron plus rimsulfuron plus atrazine POST controlled common ragweed more effectively than the PRE treatment alone. Addition of atrazine to mesotrione improved common ragweed control by at least 38 percentage points over mesotrione alone. Nicosulfuron plus rimsulfuron plus atrazine at the registered rate and in mixtures with mesotrione controlled morningglory species (pitted and ivyleaf morningglory) 89 to 91%. Large crabgrass control varied between 2002 and 2003. In 2002, large crabgrass control was 58 to 76% with all POST treatments, but in 2003, nicosulfuron plus rimsulfuron plus atrazine POST alone controlled large crabgrass greater than 86%. Large crabgrass was more effectively controlled by treatments with S-metolachlor plus atrazine PRE than by the total POST treatments in 2002. Giant foxtail was controlled at least 97% with nicosulfuron plus rimsulfuron plus atrazine treatments. S-metolachlor plus atrazine PRE fb nicosulfuron plus rimsulfuron plus atrazine POST controlled all weed species greater than 85%. Corn yields by total POST treatment combinations of mesotrione plus either rate of nicosulfuron plus rimsulfuron plus atrazine were comparable to S-metolachlor plus atrazine PRE alone or fb nicosulfuron plus rimsulfuron plus atrazine POST. Nomenclature: Atrazine; mesotrione; nicosulfuron; rimsulfuron; S-metolachlor; common lambsquarters, Chenopodium album L. #3 CHEAL; common ragweed, Ambrosia artemisiifolia L. # AMBEL; giant foxtail, Setaria faberi Herrm. # SETFA; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. # IPOHE; large crabgrass, Digitaria sanguinalis (L.) Scop. # DIGSA; morningglory species, Ipomoea spp. # IPOSS; pitted morningglory, Ipomoea lacunosa (L.) Roth # IPOLA; corn, Zea mays L. ‘Dekalb DKC60-09 (RR2)’, ‘Pioneer 33B51’, ‘Pioneer 33G56’. Additional index words: Reduced herbicide rates, total postemergence, sulfonylurea herbicides, triketone herbicides. Abbreviations: DAP, days after planting; fb, followed by; WATP, weeks after treatment with POST herbicides.

Horsenettle (Solanum carolinense) Control With a Field Corn (Zea mays) Weed Management Program1

Field studies were conducted at three sites in Delaware from 1997 to 1999 to evaluate fall glyphosate applications followed by postemergence (POST) field corn herbicides on horsenettle control and shoot densities. The fall treatments were either no fall treatment or 2.2 kg ai/ha glyphosate as a preharvest treatment in soybean at least 2 wk prior to frost. POST treatments were applied the following spring in no-tillage field corn 4 wk after planting (WAP) and included glyphosate, CGA 152005 plus primisulfuron plus dicamba, halosulfuron plus dicamba, atrazine plus dicamba, nicosulfuron plus rimsulfuron plus atrazine plus dicamba, or nicosulfuron plus dicamba. The fall glyphosate application delayed horsenettle shoot emergence in the spring and resulted in > 90% control at the time of POST in-crop applications. At 11 WAP, the highest horsenettle control was observed with a fall glyphosate application followed by POST in-crop treatments of CGA 152005 plus primisulfuron plus dicamba, halosulfuron plus dicamba, or nicosulfuron plus rimsulfuron plus dicamba and no fall treatment followed by CGA 152005 plus primisulfuron plus dicamba. Nomenclature: Atrazine; CGA 152005, 1-(4-methoxy-6-methyl-triazin-2-yl)-3-[2-(3,3,3-trifluoropropyl)-phenylsulfonyl]-urea, proposed common name prosulfuron; dicamba; glyphosate; halosulfuron; nicosulfuron; primisulfuron; rimsulfuron; horsenettle, Solanum carolinense L. #3 SOLCA; corn, Zea mays L.; soybean, Glycine max (L.) Merr. Additional index words: Integrated weed management, perennial weed control, preharvest herbicide application. Abbreviations: COC, crop oil concentrate; NIS, nonionic surfactant; POST, postemergence; PRE, preemergence; UAN, urea ammonium nitrate; WAP, weeks after planting.

Mesotrione Combinations with Atrazine and Bentazon for Yellow and Purple Nutsedge (Cyperus Esculentus and C. Rotundus) Control in Corn

Field and greenhouse studies were conducted to evaluate mesotrione alone and in combinations with low rates of atrazine and bentazon for control of yellow and purple nutsedge. Mesotrione alone at rates of 105 to 210 g ai/ha controlled yellow nutsedge 43 to 70%. Mixtures of mesotrione with atrazine at 280 g ai/ha did not always improve yellow nutsedge control over that by mesotrione alone, but increasing atrazine to 560 g ai/ha in these mixtures generally provided more consistent control of yellow nutsedge. Mesotrione at 105 g ai/ha mixed with bentazon at 280 or 560 g ai/ha controlled yellow nutsedge 88% or greater which was similar to control from the standard halosulfuron at 36 g ai/ha. Mesotrione, atrazine, and bentazon alone did not control purple nutsedge. Mixtures of mesotrione plus bentazon, however, did improve control of purple nutsedge over either herbicide applied alone, but this control was not considered commercially acceptable. Nomenclature: Atrazine; bentazon; halosulfuron; mesotrione; yellow nutsedge, Cyperus esculentus L. CYPES; purple nutsedge, Cyperus rotundus L. CYPRO; corn,Zea mays L

Weed control and potato (Solanum tuberosum) tolerance with dimethenamid isomers and other herbicides

Two research studies were conducted to evaluate weed control in potato with dimethenamid and dimethenamid-p. No significant injury was observed from most applications of dimethenamid prior to potato emergence, but injury was 20% to 38% with dimethenamid when emerging potatoes were covered slightly by soil during “drag-off” and rain occurred within 24 h. Common lambsquarters (Chenopodium album L.) and common ragweed (Ambrosia artemisiifolia L.) control with dimethenamid preemergence (PRE) did not exceed 68%. Dimethenamid-p plus metribuzin or dimethenamid-p followed by (fb) rimsulfuron postemergence (POST) controlled common lambsquarters 95% to 96% and common ragweed 71% to 92%. Annual grass control was greater with S-metolachlor alone than with dimethenamid isomers alone. Broadleaf and grass control was similar with dimethenamid and dimethenamid-p.ResumenSe ejecutaron dos trabajos de investigación para evaluar dimethenamida y dimethenamida-p en el control de malezas. No se observaron daños significativos con la mayoría de las aplicaciones de dimethenamida antes de la emergencia de la papa, pero el daño fue de 20 a 38% cuando las papas en emergencia estuvieron ligeramente cubiertas con suelo como consecuencia del rastrillado y lluvia dentro de las 24 horas. El control del bledo (Chenopodium album L.) y de la ambrosia común (Ambrosia artemisiifolia L.) con aplicación de dimethenamida a la pre-emergencia (PRE) no excedió el 68%. La aplicación de dimethenamida-p más metribuzina o de dimethamida-p seguida de rimsulfuron (fb) en postemergencia (POST), controló 95% a 96% de ambrosia y 71% a 92% de bledo. El control de pasto anual fue mayor con S-metolacloro solo que con isómeros del dimethenamida solos. El control de malezas de hoja ancha fue el mismo con dimethenamida que con methenamida-p.

Responses of Selected Weeds and Glyphosate-Resistant Cotton and Soybean to Two Glyphosate Salts1

Studies were conducted in 2000 and 2001 to investigate responses of glyphosate-resistant cotton, glyphosate-resistant soybean, and selected weed species to postemergence applications of isopropylamine (Ipa) and diammonium (Dia) salts of glyphosate at selected rates ranging from 0.42 to 3.36 kg ae/ha. No differences were detected between either glyphosate salts or application timings for cotton injury, cotton lint yield, micronaire, fiber length, fiber strength, or fiber uniformity. In a weed-free soybean study, no differences in soybean injury occurred between early-postemergence treatments of the two glyphosate salts. Injury from late-postemergence treatments did not exceed 12% with glyphosate-Ipa or 9% with glyphosate-Dia at 3.36 kg/ha. Soybean treated with glyphosate-Ipa yielded 3,050 kg/ha, whereas soybean treated with glyphosate-Dia yielded 2,880 kg/ha, when averaged over glyphosate rate and application timing. In a soybean study that included weed control as a variable, weed control at 14 d after treatment (DAT), and soybean yield was independent of glyphosate salt. Control of common ragweed, ivyleaf morningglory, pitted morningglory, and large crabgrass at 28 DAT was similar at 0.84 kg/ha of either glyphosate salt. Nomenclature: Glyphosate; common ragweed, Ambrosia artemisiifolia L. #3 AMBEL; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. # IPOHE; large crabgrass, Digitaria sanguinalis (L.) Scop. # DIGSA; pitted morningglory, Ipomoea lacunosa L. # IPOLA; cotton, Gossypium hirsutum L. ‘PM1218 BG/RR’; soybean, Glycine max (L.) Merr. ‘Asgrow 5401RR’. Additional index words: Diammonium, glyphosate formulation, glyphosate salt, isopropylamine. Abbreviations: DAT, days after treatment; Dia, diammonium; EPOST, early postemergence; Ipa, isopropylamine; LPOST, late postemergence; POST, postemergence; PRE, preemergence; Tms, trimethylsulfonium.

Effect of Fall Herbicide Treatments and Stage of Horsenettle (Solanum carolinense) Senescence on Control1

Two field studies evaluating horsenettle control were conducted from 1997 to 1999 to examine the efficacy of various fall-applied herbicides and rates and to evaluate the effect of the stage of horsenettle senescence on the effectiveness of fall glyphosate applications. The herbicides and rates evaluated in the fall herbicide efficacy study included 1.1, 2.2, or 3.4 kg ai/ha glyphosate, 1.7, 2.2, or 3.4 kg ai/ha glyphosate-trimesium, 0.6, 1.1, or 2.2 kg ai/ha dicamba, 0.06 plus 0.16, 0.09 plus 0.2, or 0.17 plus 0.44 kg ai/ha BAS 654 plus dicamba, respectively, or 2.2 kg ai/ha glyphosate plus 0.6 kg ai/ha dicamba. The highest horsenettle control in the following spring was observed with all rates of glyphosate or glyphosate-trimesium, the highest rate of BAS 654 plus dicamba, or glyphosate plus dicamba. In the study on the horsenettle stage of senescence, 2.2 kg ai/ha glyphosate was applied at stages of senescence in the fall. The presenescence stage reduced horsenettle shoot density from fall to spring and provided the highest level of control in June and July of the following year compared with plants that had already begun leaf color change and leaf drop. Nomenclature: BAS 654, 2-(1-[([3,5-difluorophenylamino]carbonyl)-hydrazono]ethyl)-3-pyridinecarboxylic acid, proposed common name diflufenzopyr; dicamba; glyphosate; horsenettle, Solanum carolinense L. #3 SOLCA. Additional index words: Fall herbicide applications, perennial weed management. Abbreviations: COC, crop oil concentrate; NIS, nonionic surfactant; POST, postemergence; UAN, urea ammonium nitrate; WAP, weeks after planting.

Evaluation of S-Metolachlor and S-Metolachlor Plus Atrazine Mixtures with Mesotrione for Broadleaf Weed Control in Corn

Abstract Field experiments were conducted in 2001, 2002, and 2003 to evaluate PRE applications of mesotrione at 150, 230, and 310 g ai/ha alone, and in mixtures with S-metolachlor at 1,070 g ai/ha and atrazine at 560 and 1,120 g ai/ha in corn. Corn injury was 11 to 18% with all treatments in 2002 when 3.2 cm of rainfall occurred within 10 d after PRE applications, but no injury was observed in 2001 and 2003 when rainfall was 0 and 1.1 cm within 10 d after PRE applications, respectively. Rainfall following PRE herbicide applications also influenced weed control, where weed control was generally poor with all herbicide treatments in 2001. Mesotrione at 150 g/ha controlled common lambsquarters and smooth pigweed at least 95% in 2002 and 2003, but control was 70% or less in 2001. PRE mesotrione at rates of 230 or 310 g/ha controlled common ragweed at least 83% in 2002 and 2003, but control exceeded 88% with mixtures of mesotrione at rates greater than 150 g/ha plus S-metolachlor plus atrazine at 560 g/ha. Morningglory species (ivyleaf morningglory, pitted morningglory, and tall morningglory) were not consistently controlled by mesotrione alone. In 2002 and 2003, mixtures of all mesotrione rates plus S-metolachlor plus atrazine at 1,120 g/ha controlled morningglory species at least 90%. Corn treated with mesotrione at any rate plus S-metolachlor plus atrazine at 1,120 g/ha consistently produced high yields. It is concluded that control with this three-way mixture would be most consistent with a minimum rate of mesotrione at 230 g/ha and atrazine at 1,120 g/ha. Nomenclature: Atrazine; mesotrione; S-metolachlor; common lambsquarters, Chenopodium album L. CHEAL; common ragweed, Ambrosia artemisiifolia L. AMBEL; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. IPOHE; morningglory species, Ipomoea spp. IPOSS; pitted morningglory, Ipomoea lacunosa L. IPOLA; smooth pigweed, Amaranthus hybridus L. AMACH; tall morningglory, Ipomoea purpurea (L.) Roth PHBPU; corn, Zea mays L. ‘Dekalb DKC64-10 (RR2)’, ‘Pioneer 33B51’, ‘Pioneer 33G26’.

Impact of Soybean Leaf Interference and Row Spacing on Preharvest Glyphosate Application1

Preharvest applications of glyphosate can be useful in controlling perennial weeds. Experiments were conducted from 1996 to 1999 to determine whether preharvest glyphosate applications are affected by differences in the amount of soybean canopy present at the time of application by measuring spray deposition and subsequently horsenettle or Canada thistle control. Soybean leaf interference levels were achieved by use of three soybean cultivars with different maturity groups to achieve no leaf interference, moderate leaf interference, and maximum leaf interference, and soybean was planted in three row spacings ranging from 19 to 76 cm. As soybean leaf interference increased, spray coverage of spray deposition cards decreased. There was a similar trend for relative spray volume, determined by intensity of the color change with water-sensitive cards. Row spacing did not influence spray coverage or relative spray volume. Percent change in horsenettle or Canada thistle stems from fall to spring counts was inconsistent. Differences detected in spray coverage did not influence weed control or weed stem density the following spring. Nomenclature: Glyphosate; Canada thistle, Cirsium arvense (L.) Scop. #3 CIRAR; horsenettle, Solanum carolinense L. # SOLCA; soybean, Glycine max (L.) Merr. Additional index words: CIRAR, Cirsium arvense, cultural practices, integrated weed management, perennial weed control, Solanum carolinense, SOLCA, spray deposition.