Characterizing geographic diversity of acetolactate synthase resistance in US johnsongrass ( Sorghum halepense ) biotypes

BACKGROUND Acetolactate synthase (ALS)‐inhibiting herbicides are widely used for Sorghum halepense weed control across agroecosystems. However, little is known about variation in resistance phenotypes and mechanisms among geographically diverse biotypes. This study surveyed S. halepense biotypes collected across the USA to quantify resistance to ALS inhibitors, identify target and non‐target site mechanisms, and evaluate alternative herbicide options. RESULTS Dose response assays identified nicosulfuron resistance in 24% of evaluated biotypes, with resistant biotypes ED 50 ranging from 66 to >4160 g ha −1 . Among nicosulfuron resistant biotypes, 56% and 44% were also resistant to imazethapyr and thiencarbazone, respectively, demonstrating variability in resistance patterns among ALS herbicides of different chemical families. A novel primer set enabled simultaneous screening of all reported ALS resistance mutations across both ALS homeologs. Molecular screening identified the Trp574 substitution as the sole target‐site resistance mechanism in 64% of resistant biotypes. Further, Trp574 mutation frequency varied among resistant biotypes, ranging from 21% to 91%, and was positively correlated with nicosulfuron resistance level. Resistant biotypes lacking target‐site mutations exhibited over eightfold reductions in nicosulfuron ED 50 following malathion pretreatment, suggesting involvement of cytochrome P450 non‐target site resistance mechanisms. Despite the identification of these mechanisms, glyphosate and clethodim remained effective for S. halepense control in all but one biotype. CONCLUSION Overall, we demonstrated that ALS resistance in S. halepense varies across geographically diverse biotypes from the USA. Together, these findings provide important insight for resistance evolution and support development of management strategies for ALS resistant S. halepense biotypes.