Suppression of Creeping Bentgrass with Sequential Applications of Plant Growth Regulators.

Golf course superintendents in the Lubbock, TX area struggle with excessive shoot growth of creeping bentgrass (Agrostis stolonifera L.) during summer months that can result in scalping and extremely slow putting green speeds.  Plant growth regulators (PGRs) are a common management tool on golf course putting greens to limit shoot elongation.  There are three commercially available chemical PGRs; trinexapac-ethyl (Primo Maxx), flurprimidol (Cutless MEC), and paclobutrazol (Trimmit 2SC).  Trinexapac-ethyl inhibits gibberellic acid (GA) production late in the pathway, and is considered the industry standard.  The other two chemistries inhibit GA early in the pathway.  Newer, commercially available PGRs (Legacy and Musketeer) have combined two or more of these chemistries.  Legacy contains trinexapac-ethyl and flurprimidol, whereas Musketeer combines all three chemistries.  The effectiveness of trinexapac-ethyl to suppress shoot growth of creeping bentgrass is reduced during summer months as temperatures rise through the transition zone.  The early GA inhibitors and combination products have not been evaluated extensively for their ability to suppress shoot growth.  The objectives of this study were to determine the growth suppression potential of sequential applications of these products on creeping bentgrass putting greens in Lubbock, TX, and to evaluate putting green performance.  Treatments were applied every two weeks from May through August at Meadowbrook Golf Club (MGC) and the Rawls Golf Course (RGC) in Lubbock, TX.  Clippings were collected from each plot two times per week, oven dried, and weighed to evaluate growth suppression of each PGR.  Visual turf quality was recorded every two weeks along with digital image analysis to determine percent green cover and color.  All PGRs initially reduced shoot growth compared to untreated controls; however, as shoot growth rates increased at the end of June, the efficacy of all products was negligible.  Trimmit and Musketeer provided greater suppression for a longer period of time.  Trimmit suppressed turf an average of 39% and Musketeer suppressed turf by 32%.  These products containing paclobutrazol caused significant phytotoxicity with initial applications that resulted in unacceptable quality ratings and significantly reduced green cover.  Percent green cover of these products recovered to similar levels as controls and other treatments at MGC by June 26.  Musketeer also provided similar cover at RGC by June 26; however, trimmit did not exhibit similar green cover until July 30.  As treatments containing paclobutrazol recovered, they demonstrated darker green color than the other plots.  The darkest green cover was observed following the initial application with trimmit at 0.9028 and Musketeer at 0.8826 compared to the untreated control at 0.6705.  Dark green color of these treatments returned to similar levels of other treatments and untreated controls by June 18.  This research provided an understanding of the growth suppression potential of these PGRs and their impact on quality and playing conditions of creeping bentgrass putting greens in a semi-arid environment within the transition zone.