Annual bluegrass is one of the most problematic weeds in the turfgrass industry, exhibiting both cross-resistance and multiple-herbicide resistance. Prodiamine, pronamide, and indaziflam are commonly used preemergence herbicides for the control of this species on golf courses in the southern United States. There have been increasing anecdotal reports of annual bluegrass populations escaping control with these herbicides, but resistance has yet to be confirmed. To evaluate the response of annual bluegrass to three herbicides, populations were collected from golf courses, athletic fields, and landscape areas in Texas and Florida, and a dose-response assay was conducted on populations that were suspected to be resistant to and known to be susceptible to prodiamine, pronamide, and indaziflam. The suspected-resistant populations showed survival to prodiamine at 32 times the recommended field rate (both populations from Florida and Texas) of 736 g ai ha^–1, and to pronamide at 32 times (the Florida populations) or 16 times (the Texas populations) the recommended field rate of 1,156 g ha^–1. In contrast, the known susceptible populations attained 100% mortality at rates as low as 46 and 578 g ha^–1, respectively, from applications of prodiamine and pronamide. For indaziflam, the suspected-resistant populations showed reduced sensitivity up to the recommended field rate of 55 g ha^–1, but they were controlled when treated with a rate twice that of the field rate. Overall, annual bluegrass populations with resistance to prodiamine and pronamide, and reduced sensitivity to indaziflam (at the recommended field rate) were confirmed from golf courses in Florida and Texas. In the presence of herbicide-resistant annual bluegrass populations, especially to commonly used herbicides such as prodiamine and pronamide, turfgrass managers should adopt integrated management strategies and frequently rotate herbicide sites of action, rather than relying solely on microtubule-assembly inhibitors or cellulose biosynthesis inhibitors, to control this species.

Nomenclature: Indaziflam; prodiamine; pronamide; annual bluegrass, Poa annua L.