Benzobicyclon will be the first 4-hydroxyphenylpyruvate dioxygenase (HPPD)–inhibiting herbicide available in US rice production pending registration completion. An observation of benzobicyclon controlling weedy rice in two field trials prompted a greenhouse and field evaluation to determine if benzobicyclon would control weedy rice accessions from Arkansas, Mississippi, and southeastern Missouri. A total of 100 accessions were screened in the greenhouse and field. Percentage mortality was determined in the greenhouse, and percentage control was recorded in the field. Benzobicyclon at 371 g ai ha–1 caused at least 80% mortality of 22 accessions in the greenhouse and at least 80% control of 30 accessions in the field. For most accessions, individual plants within the accession varied in response to benzobicyclon. Based on these results, the sensitivity of weedy rice to benzobicyclon varies across accessions collected in the midsouthern United States, and it may provide an additional control option for weedy rice in some fields.
Since the commercialization and widespread adoption of dicamba-tolerant (DT) soybean cultivars across the United States, numerous cases of off-target damage to non-DT soybean have been reported. Soybean is naturally highly sensitive to dicamba, a synthetic auxin herbicide. Previous studies have focused on understanding the impact of growth stage, dosage, frequency, and duration of dicamba exposure on the severity of symptomology and yield loss. To date, little research has investigated the effect of genetic components in the observed responses. Therefore, this study was conducted to estimate yield losses caused by prolonged off-target dicamba exposure and to identify genotypes with varying responses to off-target damage. A total of 553 soybean genotypes derived from 239 unique biparental populations were evaluated in nine environments over 3 yr. A yield penalty of 8.8% was observed for every increment in damage score on a 1-4 scale with losses as high as 40%. Although the interaction between damage and maturity group (MG) significantly affected yield, genotypes showing the most tolerance had similar yields independent of their MG. This indicated that natural tolerance to off-target dicamba may be conferred by physiological mechanisms other than the length of the recovery window. Given the widespread adoption of DT systems and potential yield losses in non-DT soybean genotypes, identification of non-DT soybean genotypes with higher tolerance to off-target dicamba may sustain and improve the production of other non-DT herbicide soybean production systems, including the niche markets of organic and conventional soybean.
BACKGROUND Protoporphyrinogen IX oxidase 2 (PPO2) inhibitors are important for the management of glyphosate‐ and acetolactate synthase‐resistant Palmer amaranth [Amaranthus palmeri (S.) Wats.]. The evolving resistance to PPO inhibitors is of great concern. We surveyed the evolution of resistance to fomesafen in the US Mid‐south and determined its correlation with the known functional PPO2 target‐site mutations (TSM). RESULTS The 167 accessions analyzed were grouped into five categories, four resistant (147) and one susceptible (20). Arkansas accessions constituted 100% of the susceptible group while the Missouri accessions comprised 60% of the most resistant category. The majority of Mississippi accessions (88%) clustered in the high‐survival‐high‐injury category, manifesting an early‐stage resistance evolution. One hundred and fifteen accessions were genotyped for four known TSMs; 74% of accessions carried at least one TSM. The most common single TSM was ΔG210 (18% of accessions) and the predominant double mutation was ΔG210 + G399A (17%). Other mutations are likely less favorable, hence are rare. All TSMs were detected in three accessions. Further examination revealed that 9 and two individuals carried G399A + G210 and G399A + R128G TSM in the same allele, respectively. The existence of these combinations is supported by molecular modeling. CONCLUSIONS Resistance to PPO inhibitors is widespread across the Mid‐southern USA. Highly resistant field populations have plants with multiple mutations. G399A is the most prone to co‐occur with other ppo2 mutations in the same allele. Mutation at R128 in the configuration of the PPO2 catalytic domain restrains the co‐occurrence of R128G with ΔG210, making ΔG210 + G399A the most plausible, tolerable functional mutation combination to co‐occur in the same ppo2 allele.
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