These studies confirm that waterhemp has evolved resistance to HPPD-inhibiting herbicides.

A waterhemp population (McLean County resistant, MCR) from McLean County, Illinois is resistant to both mesotrione and atrazine by elevated rates of herbicide metabolism. Research was conducted to investigate the inheritance of these resistance traits. Resistant and sensitive plants were crossed to obtain reciprocal F1populations, which were then used to create pseudo-F2and backcross (to sensitive parent; BCS) populations. The various populations were evaluated with whole-plant herbicide efficacy studies in a greenhouse. The responses of the F1populations to both mesotrione and atrazine were intermediate when compared with parental populations. In the case of atrazine, BCSand F2populations segregated 1 : 1 and 1 : 3, respectively, for susceptibility (S) : resistance (R), at a dose that controlled the sensitive parent but not the F1or resistant parent. For mesotrione, variability was observed within the F1populations, suggesting that mesotrione resistance is multigenic and the resistant parents used in the cross were not homozygous at the resistance loci. Furthermore, at low mesotrione doses, more F2plants survived than expected on the basis of a single-gene trait, whereas at high doses, fewer F2plants survived than expected. Dry weight data confirmed the conclusions obtained from survival data. Specifically, atrazine responses segregated into two discrete classes (R and S) in both the F2and BCSpopulations, whereas mesotrione responses showed continuous distributions of phenotypes in F2and BCSpopulations. We conclude that metabolism-based atrazine resistance in MCR is conferred by a single major gene, whereas inheritance of mesotrione resistance in this population is complex.

Field experiments were conducted in 2010 and 2011 at a Mclean County, IL seed corn production field where resistance to foliar-applied 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors was confirmed in waterhemp. Corn herbicides were applied to the soil at 1 and 2 times (1× and 2×, respectively) the recommended field use rate, while soybean herbicides were applied only at 1× the recommended rate. Waterhemp control and density were determined 30 and 60 d after treatment (DAT). In corn, 1× rates of mesotrione, safened and unsafened isoxaflutole formulations, atrazine, andS-metolachlor provided less than 70% control 30 DAT, while control with acetochlor was greater than 80%. One and 2× rates of acetochlor and 2× rates of mesotrione and unsafened isoxaflutole provided the greatest reduction of waterhemp density across years. At 30 DAT in soybean, sulfentrazone, flumioxazin, metribuzin, and pyroxasulfone provided the highest levels of waterhemp control (84 to 92%), as well as the greatest reduction in waterhemp density both years. A dose–response experiment with soil-applied mesotrione was performed under controlled greenhouse conditions using three waterhemp populations: MCR15 (seed collected from the McLean Co. site), NH41 (progeny obtained from the McLean Co. population by an additional generation of mesotrione selection in the greenhouse), and a sensitive (S). Emergence counts 21 DAT revealed higher seedling survival of MCR15 and NH41 at mesotrione rates of 105 g ha−1or less compared with the sensitive control. Resistant-to-sensitive (R/S) ratios for NH41 and MCR15 were 12.7 and 8.8, respectively. Field results indicate the McLean Co. waterhemp population demonstrates reduced sensitivity to soil-applied HPPD-inhibiting herbicides. This is supported by greenhouse results that demonstrate reduced sensitivity to mesotrione in MCR15 and NH41.

A waterhemp population (MCR) previously characterized as resistant to 4-hydroxyphenylpyruvate dioxygenase and photosystem II inhibitors demonstrated both moderate and high levels of resistance to acetolactate synthase (ALS) inhibitors. Plants from the MCR population exhibiting high resistance to ALS inhibitors contained the commonly found Trp574Leu ALS amino acid substitution, whereas plants with only moderate resistance did not have this substitution. A subpopulation (JG11) was derived from the MCR population in which the moderate-resistance trait was isolated from the Trp574Leu mutation. Results from DNA sequencing and ALS enzyme assays demonstrated that resistance to ALS inhibitors in the JG11 population was not due to an altered site of action. This nontarget-site ALS-inhibitor resistance was characterized with whole-plant dose–response experiments using herbicides from each of the five commercialized families of ALS-inhibiting herbicides. Resistance ratios ranging from 3 to 90 were obtained from the seven herbicides evaluated. Nontarget-site resistance to ALS has been rarely documented in eudicot weeds, and adds to the growing list of resistance traits evolved in waterhemp.

Field and greenhouse experiments were conducted to characterize the response of a waterhemp population from McLean County, IL to foliar-applied 4-hydroxyphenylpyruvate dioxygenase (HPPD) -inhibiting herbicides and determine the population's sensitivity to herbicides from other site-of-action groups. In the field, 10 to 15-cm-tall waterhemp treated with mesotrione at 105 g ai ha À1 , tembotrione at 92 g ai ha À1 , or topromezone at 18 g ai ha À1 had significantly greater biomass (! 10%) 14 d after treatment (DAT) than waterhemp harvested the day of herbicide application, indicating growth had occurred following herbicide application. Waterhemp growth stage at the time of herbicide application influenced control. Mesotrione applied at 105 g ha À1 alone or combined with atrazine at 560 g ai ha À1 provided significantly greater waterhemp control (! 66%) when applied to small waterhemp plants (2 to 5 cm tall) compared with applications made to plants 5 to 10 or 10 to 15 cm tall. Glyphosate, glufosinate, fomesafen, lactofen, or acifluorfen provided greater waterhemp control (! 68%) 7 and 14 DAT than mesotrione, dicamba, or 2,4-D. Control of this population with atrazine, chlorimuron, and imazethapyr did not exceed 12%. Results of a greenhouse experiment with waterhemp plants grown from field-collected seed were similar to field data, and confirm the McLean County population was poorly controlled with HPPD, photosystem II, and acetolactate synthase inhibitors. Nomenclature: Acifluorfen; atrazine; chlorimuron; dicamba; fomesafen; glufosinate; glyphosate; imazethapyr; lactofen; mesotrione; tembotrione; topramezone; 2,4-D; waterhemp, Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea and Tardif AMATU. Key words: Herbicide resistance, waterhemp management.Experimentos de campo e invernadero fueron realizados para caracterizar la respuesta de una población de Amaranthus tuberculatus proveniente del condado McLean en Illinois, a la aplicación foliar de herbicidas inhibidores de 4-hydroxyphenylpyruvate dioxygenase (HPPD) y determinar la sensibilidad de la población a herbicidas de grupos con otros sitios de acción. En el campo, plantas de A. tuberculatus de 10 a 15 cm de altura, tratadas con mesotrione a 105 g ai ha À1 , tembotrione a 92 g ai ha À1 , o topramezone a 18 g ai ha À1 , tuvieron una biomasa significativamente mayor (!10%) 14 d después del tratamiento (DAT) que A. tuberculatus cosechado el día de la aplicación del herbicida, indicando que hubo crecimiento después de la aplicación del herbicida. El estadio de desarrollo de A. tuberculatus al momento de la aplicación del herbicida influyó en el control. Mesotrione aplicado solo a 105 g ha À1 o combinado con atrazine a 560 g ai ha À1 brindó un control significativamente mayor (!66%) cuando se aplicó a plantas pequeñas de A. tuberculatus (2 a 5 cm de altura), al compararse con aplicaciones hechas a plantas de 5 a 10ó 10 a 15 cm de altura. Glyphosate, glufosinate, fomesafen, lactofen, o acifluorfen brindaron mayor control de A. tuberculatus (!68%) 7 y 14 DAT...