Cross-resistance of Echinochloa species to acetolactate synthase inhibitor herbicides

This study was conducted to evaluate the cross-resistance of acetolactate synthase (ALS) inhibitors with different chemistries, specifically azimsulfuron (sulfonylurea), penoxsulam (triazolopyrimidine sulfonanilide) and bispyribac-sodium (pyrimidinyl thio benzoate), in Echinochloa oryzicola and Echinochloa crus-galli that had been collected in South Korea and to investigate their herbicide resistance mechanism. Both Echinochloa spp. showed cross-resistance to the ALS inhibitors belonging to the above three different chemistries. In a whole plant assay with herbicides alone, the resistant/susceptible ratios for azimsulfuron, penoxsulam and bispyribac-sodium were 12.6, 28.1 and 1.9 in E. oryzicola and 21.1, 13.7 and 1.8 in E. crus-galli, respectively. An in vitro ALS enzyme assay with herbicides showed that the I ₅₀-values of the resistant accessions were approximately two-to-three times higher than the susceptible accessions, with no statistical difference, suggesting that the difference in ALS sensitivity cannot explain ALS inhibitor resistance in Echinochloa spp. for azimsulfuron, penoxsulam and bispyribac-sodium. A whole plant assay with fenitrothion showed that the GR ₅₀-values significantly decreased in both the resistant E. oryzicola and E. crus-galli accessions when azimsulfuron, penoxsulam and bispyribac-sodium were applied with the P450 inhibitor, while no significant decrease was observed in the susceptible accessions when the P450 inhibitor was used. Thus, these results suggest that ALS inhibitor cross-resistance for azimsulfuron, penoxsulam and bispyribac-sodium is related to enhanced herbicide metabolism.