Triazine-Resistant Annual Bluegrass ( Poa annua ) Populations with Ser 264 Mutation Are Resistant to Amicarbazone

Amicarbazone is a photosystem II (PSII)-inhibiting herbicide in the triazolinone family, which is similar in mode of action to the triazines. Annual bluegrass is a cool-season weed and has shown resistance to some PSII-inhibiting herbicides. The objective was to evaluate triazine-resistant and -susceptible annual bluegrass populations for potential cross-resistance to amicarbazone. Two triazine-resistant (MS-01, MS-02) and triazine-susceptible (AL-01, COM-01) annual bluegrass populations were treated with amicarbazone, atrazine, and simazine at 0.26, 1.7, and 1.7 kg ai ha −1 , respectively. All herbicide treatments controlled the susceptible populations greater than 94% 2 wk after treatment (WAT). No visual injury of MS-01 and MS-02 was observed at any time following herbicide treatment. Quantum yield (Φ PSII ) of annual bluegrass was measured 0 to 72 h after application (HAA) to determine the photochemical effects of amicarbazone compared to other PSII inhibitors. Φ PSII of triazine-susceptible populations was reduced at all measurement times by all three herbicides. However, amicarbazone decreased Φ PSII of susceptible populations faster and greater than atrazine and simazine at most measurement times. Amicarbazone did not reduce Φ PSII of the MS-01 population. Amicarbazone significantly reduced Φ PSII of the MS-02 population during several measurement timings; however, these reductions were short-lived compared to the susceptible populations and no trend in Φ PSII reduction was observed. Sequencing of the psbA gene revealed a Ser to Gly substitution at amino acid position 264 known to confer resistance to triazine herbicides. These data indicate amicarbazone efficiently inhibited PSII of susceptible annual bluegrass populations; however, triazine-resistant annual bluegrass populations with Ser 264 to Gly mutations are cross-resistant to amicarbazone.