Seasonal Factors Affecting the Control of Annual Bluegrass (Poa annua L.) with Xanthomonas campestris pv. poae (JT-P482)

Abstract To ascertain the optimum timing for the application of Xanthomonas campestris pv. poae to control annual bluegrass ( Poa annua ), the differences in the efficacy of treatments initiated in consecutive months were investigated. October treatment and February through April treatments produced acceptable levels of fresh weight (FW) loss in annual bluegrass the following spring in pot experiments set in the field. More than 75% cumulative seedhead suppression was achieved by the October, November, and February treatments. Bacterial populations in annual bluegrass plants increased to 10 9 CFU/g of FW in the case of all application timings but the best control was obtained when maximum bacterial numbers were reached within 1 month of treatment. The differences in efficacy caused by different application timings (fall or spring) were compared in different turf fields. In two Kohrai ( Zoysia matrella ) turfs and in Kentucky bluegrass ( Poa pratensis ) turf, the fall and spring treatments were highly effective. However, in bentgrass ( Agrostis palustris ) turf, control of annual bluegrass was slower. October and/or March treatments effectively inhibited seedhead production, but April treatment was too late to prevent seed scattering. Taking into account late-emerging annual bluegrass, the optimum strategy for timing of treatments is application in early fall to obtain significant fresh weight loss and seedhead suppression or application in early spring, just before seed ripening, to obtain seedhead suppression and to control late-emerging plants.