ABSTRACT In recent years, resistance to the ACCase inhibitor cyhalofop‐butyl and the ALS inhibitor penoxsulam has evolved in Echinochloa spp., causing serious problems in dry‐seeded rice in Japan. To clarify the current state of herbicide resistance in Japan, we developed a simple method for diagnosing herbicide resistance by germinating seeds in herbicide solutions and evaluating seedling growth. When the herbicide solution contained a standard rate of Murashige–Skoog salt mixture and a 2000‐fold dilution of a mixed liquid fungicide (30.0% hymexazol and 2.0% metalaxyl‐M), clear differences were observed in seedling growth between resistant and susceptible lines in the response to both cyhalofop‐butyl and penoxsulam. This difference became apparent 10 days after treatment and was more prominent 14 days after treatment. Under these optimal conditions, cyhalofop‐butyl at concentrations of 1.2 ppm or higher clearly distinguished between the resistant and susceptible lines of the three Echinochloa weeds: E. crus‐galli var. crus‐galli , E. crus‐galli var. formosensis , and E. oryzicola . Penoxsulam concentrations of 0.09 ppm or higher also clearly distinguished between the two lines in the three weeds. The method using 1.2 ppm cyhalofop‐butyl and 0.09 ppm penoxsulam was able to successfully diagnose two susceptible lines, three cyhalofop‐butyl‐resistant lines, and two lines with multiple herbicide‐resistance to cyhalofop‐butyl and penoxsulam. Thus, this study developed a simple diagnostic method for detecting cyhalofop‐butyl and/or penoxsulam resistance in Echinochloa spp. This method can serve as an effective and practical tool for diagnosing herbicide resistance and clarifying the resistance status of Echinochloa spp. in areas where they infest paddy fields.
Yoshikawa et al. (2026) studied this question.