Evaluation of Triazine-Resistant and Susceptible Annual Bluegrass Biotypes Following Applications of Amicarbazone and Atrazine.

Annual bluegrass (Poa annua L.) is a ubiquitous, cool-season grass species that is challenging to control in fine turf.  Herbicide-resistant annual bluegrass has been reported across multiple states and countries. Many of these reports are attributed to herbicides with a photosystem II (PSII) inhibiting mode of action. Amicarbazone is a new photosystem II inhibitor being evaluated for annual bluegrass control, and the potential for resistance is a concern. Two greenhouse studies investigated the effects of amicarbazone on two triazine-resistant and two triazine-susceptible annual bluegrass biotypes. Herbicide treatments included amicarbazone (0.26 kg/ha), atrazine (1.7 kg/ha), and simazine (1.7 kg/ha). A nontreated was included for statistical comparison. Photosynthetic yield (Φ_PSII) was measured 0, 1, 2, 4, 8, 16, 24, 48, 72, and 168 hours after application (HAA) using a pulse-modulated chlorophyll fluorometer. Plants were placed in artificial lighting 30 minutes prior to and during each measurement and placed in greenhouse growing conditions otherwise. Treatments were a factorial combination of herbicide treatment and annual bluegrass population. A significant herbicide by biotype interaction existed. Amicarbazone did not reduce Φ_PSII of either resistant population at any timing. However, amicarbazone treatment of both susceptible populations reduced Φ_PSII rapidly in both studies. Amicarbazone reduced Φ_PSII of both susceptible populations more than atrazine and simazine at 1HAA, indicating amicarbazone may be more efficient at inhibiting PSII of annual bluegrass. Amicarbazone completely controlled both susceptible populations 2 weeks after application (WAA). Amicarbazone did not injure either triazine-resistant population. Annual bluegrass response to amicarbazone treatment in this study indicates cross-resistance may exist between amicarbazone and triazine-resistant annual bluegrass.