Baseline Sensitivity of Echinochloa crus-galli (L.) P.Beauv. and Leptochloa chinensis (L.) Nees to Flusulfinam, a New 4-Hydroxyphenylpyruvate Dioxygenase (HPPD)-Inhibiting Herbicide in Rice, in China

Flusulfinam is a 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide applied post-emergence (POST) to control <i>Echinochloa crus-galli</i> (L.) P.Beauv., <i>Leptochloa chinensis</i> (L.) Nees, <i>Digitaria sanguinalis</i> (Linn.) Scop. and other annual weeds in directly seeded and transplanted paddy fields in China, registered in September 2024. Notably, compared with other HPPD inhibitors in rice, flusulfinam exhibits consistently high safety in both japonica and indica rice varieties. Meanwhile, flusulfinam has no target-site cross-resistance with traditional acetolactate synthase (ALS)-inhibiting, acetyl-CoA carboxylase (ACCase)-inhibiting, and auxin herbicides. Moreover, as the only heterocyclic-amide-structured herbicide in the HPPD inhibitors, it poses a low risk of metabolic cross-resistance with the other HPPD inhibitors, making it a promising candidate for managing herbicide-resistant weeds in rice fields. In this study, the baseline sensitivity to flusulfinam of <i>E. crus-galli</i> and <i>L. chinensis</i> in paddy fields in China was established using dose-response assays between June and October 2023. Thirty-nine populations of <i>E. crus-galli</i> and forty-three populations of <i>L. chinensis</i>, collected from rice fields across various major rice-producing regions in China, exhibited susceptibility to flusulfinam. The GR₅₀ values ranged from 0.15 to 19.39 g active ingredient (a.i.) ha^-1 for <i>E. crus-galli</i> and from 7.82 to 49.92 g a.i. ha^-1 for <i>L. chinensis</i>, respectively, far below the field recommended rate of flusulfinam. Meanwhile, the GR₅₀ values of <i>E. crus-galli</i> and <i>L. chinensis</i> to flusulfinam were both distributed as a unimodal curve, with baseline sensitivity (GR₅₀b) of 6.48 g a.i. ha^-1 and 22.38 g a.i. ha^-1, respectively. The SI₅₀ value showed 129.27-fold and 6.38-fold variability in flusulfinam sensitivity among the 39 <i>E. crus-galli</i> field populations and 43 <i>L. chinensis</i> filed populations, while the variability declined to 2.99-fold and 2.23-fold when the SI₅₀b value was used. This study substantiated the efficacy of flusulfinam against <i>E. crus-galli</i> and <i>L. chinensis</i> in Chinese paddy fields and furnished a benchmark for monitoring temporal variations in the susceptibility of field populations of <i>E. crus-galli</i> and <i>L. chinensis</i> to flusulfinam.