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 (ΦPSII) of annual bluegrass was measured 0 to 72 h after application (HAA) to determine the photochemical effects of amicarbazone compared to other PSII inhibitors. ΦPSII of triazine-susceptible populations was reduced at all measurement times by all three herbicides. However, amicarbazone decreased ΦPSII of susceptible populations faster and greater than atrazine and simazine at most measurement times. Amicarbazone did not reduce ΦPSII of the MS-01 population. Amicarbazone significantly reduced ΦPSII of the MS-02 population during several measurement timings; however, these reductions were short-lived compared to the susceptible populations and no trend in ΦPSII 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 Ser264 to Gly mutations are cross-resistant to amicarbazone.
Nomenclature: Amicarbazone, atrazine, simazine, annual bluegrass, Poa annua L.