Influence of Methiozolin Rates and Application Timings On Poa Annua Populations.

Methiozolin is an experimental post-emergent herbicide designed to selectively control annual bluegrass (Poa annua L.; ABG).  The objective of this study was to evaluate the herbicide’s efficacy to control ABG when applied at two rates and initiated at three seasonal timings.  A field study was initiated in 2011 on a golf course putting green with a mixed stand of creeping bentgrass (Agrostis stolonifera L.; CBG) (~50%) and ABG (~50%) located in State College, PA.  Plots measured 0.9 x 1.8 m and were set up as a 2 x 3 incomplete factorial and arranged in a randomized complete block design with four replications.  Main factors included methiozolin rates (0.5 kg a.i. ha^-1 and 1.0 kg a.i. ha^-1) and timings (spring, summer and autumn).  Treatments were initiated on 4 May, 15 June or 31 Aug and applied every 2 weeks until the total herbicide load reached 3.0 kg a.i. ha^-1.  On 4 Nov 2011, turf within plots treated with methiozolin, regardless of timing or rate, had < 10% living ABG while the untreated plots had 31%.  When plots were rated on 18 Apr 2012, however, autumn applications of methiozolin resulted in the least percent ABG (1 to 3%) when compared to all other treatments.  Few differences in percent ABG existed between methiozolin treatments applied in the summer (56 to 57%) and the untreated control (65%).  Moderate percentages of ABG were observed within plots treated with methiozolin in the spring and differences in percent ABG between the 0.5 kg a.i. ha^-1 rate (46%) and 1.0 kg a.i. ha^-1 rate (28%) were significant.  Although ABG suppression was possible, overall turfgrass quality declined in select treatments and injury was moderate to severe at times.  Methiozolin rates and application timings should be considered when a slow and non-invasive conversion to CBG is desired.