Seasonal factors affecting the control of annual bluegrass (Poa annua L.) with Xanthomonas campestris pv. poae (JT-P482)

To ascertain the optimum timing for the application of Xanthomonas campestr is 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 held. More than 75% cumulative seedhead suppr ession was achieved by the October, November, and February treatments. Bact erial 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 w hen maximum bacterial numbers were reached within 1 month of treatment. The differences in efficacy caused by different application timings (fall or s pring) were compared in different turf fields. In two Kohrai (Zoysia matrel la) turfs and in Kentucky bluegrass (Poa pratensis) turf, the fall and spri ng treatments were highly effective. However, in bentgrass (Agrostis palust ris) turf, control of annual bluegrass was slower. October and/or March tre atments effectively inhibited seedhead production, but April treatment was too late to prevent seed scattering. Taking into account late-emerging annu al bluegrass, the optimum strategy for timing of treatments is application in early fall to obtain significant fresh weight loss and seedhead suppress ion or application in early spring, just before seed ripening, to obtain se edhead suppression and to control late-emerging plants. (C) 1999 Academic P ress.