Florasulam resistance of shepherd's purse ( <i>Capsella bursa‐pastoris</i> ) and candidate herbicides for its chemical control in wheat fields of North China plain

Abstract Shepherd's purse ( Capsella bursa‐pastoris ) is a widespread weed in wheat fields across China. In recent years, the efficacy of florasulam, a widely used acetolactate synthase (ALS) inhibitor for broad‐leaved weed control, has diminished in wheat fields of the North China plain, even leading to control failures. However, the herbicide resistance status of shepherd's purse in wheat fields of these regions remains unclear, posing a significant threat to the sustainable management of this weed and necessitating urgent attention in agricultural practices. Therefore, 47 shepherd's purse populations were sampled from wheat fields in these regions, their florasulam resistance and ALS resistance mutations were identified. Among them, five populations were classified as susceptible (S), nine populations exhibited a potential risk of developing resistance (R?) and 33 populations were resistant (R), with resistance indices ranging from 3.9 to 89.2‐fold. Additionally, three ALS resistance mutations of Pro197S/L/T were identified in the resistant populations, which displayed reduced sensitivity to florasulam. Cross‐resistance tests revealed that three target‐site resistance (TSR) populations (HN11, HN08 and HB02) exhibited cross‐resistance to the tested ALS inhibitors but remained sensitive to herbicides with other modes of action such as MCPA‐Na, cypyrafluone and carfentrazone‐ethy. In contrast, the other two TSR populations (HN19 and SD12) and most non‐target‐site‐based resistance (NTSR) (HN14, HN05, etc.) populations displayed relatively lower sensitivity to the other herbicides tested. These findings indicated that the co‐existence of TSR and NTSR mechanisms drives florasulam resistance in shepherd's purse. Our greenhouse pot experiment demonstrated that photosystem II inhibitors (bromoxynil octanoate) and auxin mimics with protoporphyrinogen oxidase inhibitors (mixtures of MCPA‐Na and carfentrazone‐ethyl) have excellent efficacy for controlling shepherd's purse.