UC Riverside

Methiozolin is a new herbicide from South Korea currently under development for pre- (PRE) and postemergence (POST) control of annual bluegrass in bentgrass and most other cool- and warm-season turfgrasses. Greenhouse studies were conducted in 2012 at the University of California, Riverside, CA, and Auburn University, Auburn, AL, to evaluate the relative tolerance of nine creeping bentgrass (CRBG) cultivars, velvet bentgrass (VBG) and colonial bentgrass (COBG) to methiozolin at 0, 0.6, 1.1, 2.2, 4.5 and 9.0 kg ai ha^-1. Methiozolin was applied 7 weeks after seeding followed by a second application at 12 weeks. Methiozolin rates that produced 25% injury (TI₂₅) and 50% clipping dry weight reduction relative to an untreated control for each species or cultivar (GR₅₀) were calculated using four parameter logistic regression. Results indicated that TI₂₅ rates at 56 DAIT were the most accurate in describing relative tolerance among BG species; as a result CRBG was determined to be more tolerant to methiozolin than VBG or COBG. Respective TI₂₅ rates were 1.1, 0.2, and 0.3 kg ai ha^-1 for CRBG, VBG and COBG. Furthermore, VBG and COBG are unlikely to tolerate sequential applications necessary to control annual bluegrass with methiozolin. Evidence of soil residual activity was documented for methiozolin; therefore, herbicide rates that caused TI₂₅ and GR₅₀ decreased with increasing number of applications. Creeping bentgrass `Penn A-4' was the most tolerant of methiozolin application with TI₂₅ of 4.5 and 2.5 kg ai ha^-1 at 28 and 56 DAIT, respectively. All CRBG cultivars tested tolerated methiozolin at the recommended sequential application rate (0.5 kg ai ha^-1).