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The herbicides that inhibit 4-hydroxyphenylpyruvate dioxygenase (HPPD) are primarily used for weed control in corn, barley, oat, rice, sorghum, sugarcane, and wheat production fields in the United States. The objectives of this review were to summarize 1) the history of HPPD-inhibitor herbicides and their use in the United States; 2) HPPD-inhibitor resistant weeds, their mechanism of resistance, and management; 3) interaction of HPPD-inhibitor herbicides with other herbicides; and 4) the future of HPPD-inhibitor-resistant crops. As of 2022, three broadleaf weeds (Palmer amaranth, waterhemp, and wild radish) have evolved resistance to the HPPD inhibitor. The predominance of metabolic resistance to HPPD inhibitor was found in aforementioned three weed species. Management of HPPD-inhibitor-resistant weeds can be accomplished using alternate herbicides such as glyphosate, glufosinate, 2,4-D, or dicamba; however, metabolic resistance poses a serious challenge, because the weeds may be cross-resistant to other herbicide sites of action, leading to limited herbicide options. An HPPD-inhibitor herbicide is commonly applied with a photosystem II (PS II) inhibitor to increase efficacy and weed control spectrum. The synergism with an HPPD inhibitor arises from depletion of plastoquinones, which allows increased binding of a PS II inhibitor to the D1 protein. New HPPD inhibitors from the azole carboxamides class are in development and expected to be available in the near future. HPPD-inhibitor-resistant crops have been developed through overexpression of a resistant bacterial HPPD enzyme in plants and the overexpression of transgenes for HPPD and a microbial gene that enhances the production of the HPPD substrate. Isoxaflutole-resistant soybean is commercially available, and it is expected that soybean resistant to other HPPD inhibitor herbicides such as mesotrione, stacked with resistance to other herbicides, will be available in the near future.
Herbicides registered in vegetable soybean often fail to control waterhemp. The objective of this research was to quantify vegetable soybean tolerance to preemergence herbicides for early-season waterhemp control, including flumioxazin applied alone PRE or in mixture with chlorimuron, metribuzin, or pyroxasulfone at use rates in grain-type soybean. Crop tolerance to the herbicides was tested in field trials with 20 vegetable soybean cultivars and four grain-type cultivars through 4 wk after treatment (WAT). Flumioxazin-based treatments were equally safe, resulting in only minor, transitory crop response (<5% injury 2 WAT) and no effect on crop emergence or early season growth. Flumioxazin mixtures provided greater than 99% control of waterhemp 4 WAT, as evidenced by reduced weed density from 29.7 plants m−2 in the nontreated control to no waterhemp. Flumioxazin applied alone or in tank mixture with chlorimuron, metribuzin, or pyroxasulfone were as safe in vegetable soybean as previously reported in grain-type soybean. Registration of these products in vegetable soybean would provide the industry with additional options for managing waterhemp.
If registered for use on vegetable soybean, pyroxasulfone would expand the options for weed management systems in the crop. In order to determine the potential crop injury risk of pyroxasulfone on vegetable soybean, the objective of this work was to quantify vegetable soybean tolerance to pyroxasulfone applied PRE and EPOST. Twenty-one vegetable soybean and two grain-type soybean cultivars were treated with pyroxasulfone at 417 gaiha−1 (twice the recommended field use rate) PRE, EPOST, or not treated. Plant population density was unaffected by pyroxasulfone. Only low levels (<10%) of crop injury were observed within a few weeks after PRE and EPOST treatments. Soybean cultivars were not differentially affected by pyroxasulfone, as evidenced by the lack of interactions between cultivar and treatment for any crop response variable. The low amount of risk of crop injury associated with pyroxasulfone is no different for vegetable soybean cultivars grown in the US for commercial production than grain-type soybean.
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−1, tembotrione at 92 g ai ha−1, or topromezone at 18 g ai ha−1 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−1 alone or combined with atrazine at 560 g ai ha−1 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.
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 F1 populations, which were then used to create
pseudo-F2 and backcross (to sensitive parent; BCS)
populations. The various populations were evaluated with whole-plant
herbicide efficacy studies in a greenhouse. The responses of the
F1 populations to both mesotrione and atrazine were
intermediate when compared with parental populations. In the case of
atrazine, BCS and F2 populations segregated 1 : 1 and
1 : 3, respectively, for susceptibility (S) : resistance (R), at a dose that
controlled the sensitive parent but not the F1 or resistant
parent. For mesotrione, variability was observed within the F1
populations, 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 F2 plants
survived than expected on the basis of a single-gene trait, whereas at high
doses, fewer F2 plants 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 F2 and BCS populations, whereas mesotrione
responses showed continuous distributions of phenotypes in F2 and
BCS populations. 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, and S-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−1 or 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.
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