Characterization of Mesosulfuron-Methyl + Iodosulfuron-Methyl and Pyroxsulam-Resistant Annual Bluegrass (Poa annua) in an Annual Cropping System

Abstract Herbicide-resistant annual bluegrass ( Poa annua L.) has become a problem in non-arable land areas. In arable fields, P. annua is frequently of lower priority in weed control programmes due to the variety of control options available and the relatively modest impact on crop yield than other species. In Ireland, postemergence herbicides are not primarily intended for P. annua control, but some herbicides, including acetolactate synthase (ALS)-inhibitor mesosulfuron-methyl + iodosulfuron-methyl, exhibit P. annua -activity. In this study, a suspected P. annua population (POAAN-R) that survived mesosulfuron-methyl + iodosulfuron-methyl at 0.75 of field-recommended rate was sampled from a wheat field in County Dublin, Ireland. Single-dose testing confirmed that the suspected POAAN-R had evolved resistance to mesosulfuron-methyl + iodosulfuron-methyl and additionally, to pyroxsulam (not registered in Ireland for P. annua control), but was sensitive to clethodim, glyphosate, pendimethalin and flufenacet. Dose-response experiments indicated that POAAN-R was more resistant (GR 50 resistance index) to both mesosulfuron-methyl + iodosulfuron-methyl (47.8 times) and pyroxsulam (38.0 times) than sensitive POAAN-S and this was associated with the mutation at tryptophan (Trp)-574 in ALS protein. Malathion (a cytochrome P450 inhibitor) pre-treatment did not reverse POAAN-R resistance to mesosulfuron-methyl + iodosulfuron-methyl or pyroxsulam at field rate or above. The natural inherent mutation at isoleucine (Ile)-1781 in ACCase protein had no effect on both POAAN-R and POAAN-S sensitivity to clethodim. The glyphosate sensitivity of POAAN-R also corresponded with no known mutation in EPSPS protein. Based on field histories, poor early-season weed control coupled with intensive use of mesosulfuron-methyl + iodosulfuron-methyl (often at reduced rates) has unintentionally selected for ALS-inhibitor-resistant POAAN-R. This is the first report to characterize resistance in P. annua to ALS-mesosulfuron-methyl + iodosulfuron-methyl and pyroxsulam in an arable setting. There is an opportunity to effectively control POAAN-R using herbicides, but this needs a wide-ranging and varied approach, coupled with cultural/non-chemical practices.