Acetolactate Synthase Herbicide-Resistant Stellaria media (L.) Vill. Populations Documented in Forage Cropping Systems in California

Common chickweed (Stellaria media (L.) Vill.) has been observed invading forage wheat (Triticum aestivum L.) and triticale (xTriticosecale) fields treated with acetolactate synthase (ALS)-inhibiting herbicides in the southern San Joaquin Valley (SJV) of California. Studies were conducted to verify if these weed populations were resistant to these herbicides and if so, what the mechanism of resistance was. Seeds of common chickweed plants were collected from three locations representing four populations—three suspected resistant and one suspected susceptible to ALS inhibitor herbicides. Plants were grown from these collections, and greenhouse dose–response studies were conducted twice using five common ALS inhibitors: mesosulfuron-methyl, pyroxsulam, imazamox, imazethapyr, and tribenuron-methyl. Results confirmed that three of the populations were resistant to the label rate of all five ALS-inhibitor herbicides tested in this study. Sanger sequencing identified the Trp574Leu substitution, a mutation previously associated with target-site resistance to ALS-inhibiting herbicides. This is the first case of common chickweed populations cross-resistant to ALS herbicides west of the Rocky Mountains in North America.