Suppression of shoot growth and improved putting green performance with plant growth regulators
Suppression of creeping bentgrass (Agrostis stolonifera L.) putting greens from plant growth regulators (PGRs) appears to have declined in recent years. Golf course superintendents on the Southern High Plains of Texas struggle with excessive shoot growth of creeping bentgrass during summer months that can result in reduced turf quality (non-uniform growth or scalping) and performance (slow putting green speeds). Creeping bentgrass in semi-arid regions seems to accelerate in growth as the ambient air temperature increases through the summer. Making sequential applications of PGRs to the putting greens during mid to late summer has little to no suppression of the turf. The objectives of this study were to develop growing degree day (GDD) models of commercially available PGRs and analyze visual turf quality and PGR suppression capacity with sequential applications of PGRs on creeping bentgrass putting greens. This research was conducted on two golf courses in Lubbock, TX in the summer of 2014 and 2015. Modeling PGR applications were allowed 1000 GDDs between applications and sequential applications were made every two weeks. Plant growth regulators commercially available for bentgrass putting greens at this time were trinexapac-ethyl (TE), paclobutrazol, flurprimidol, Legacy (TE and flurprimidol) and Musketeer (TE, paclobutrazol, and flurprimidol). Clippings were collected twice per week from the PGR plots and evaluated against the control plots. Turf quality was evaluated visually and using digital image analysis to calculate percent green cover and dark green color with the plots every two weeks between sequential PGR applications. Peak suppression of the turf on the modeling applications ranged from 24% with flurprimidol to 39% with Musketeer. The variation between these products resulted in an additional 83 GDD of accumulation with Musketeer compared to flurprimidol, which could represent an additional 3 to 7 days before additional applications would be required. Legacy and Musketeer suppressed the turf for a longer period than other PGRs. Turf quality was reduced with initial applications of products containing paclobutrazol due to high phytotoxicity from over suppression of the creeping bentgrass. However, those plots treated with products containing paclobutrazol or flurprimidol provided darker green color compared to the untreated control or TE alone on most dates. Sequential applications of PGRs made later in the summer had limited suppression relative to the control plots. Products containing paclobutrazol suppressed the turf more than other sequential applications of PGRs. Ultimately, this research proves that all commercially available products reduce foliar growth of creeping bengrass following application; however, the level of suppression provided or the length of time between applications differs with external factors. The temperature and growth rate of creeping bentgrass should be considered when selecting a product use rate because moderate rates of some products can result in over suppression if applied during sub-optimal growth periods. Over-suppressing the turf can lead to limited traffic tolerance and recovery or increased disease pressure in some regions. Sequential applications of PGRs made based on GDD are would likely be more consistent at suppressing the turfgrass than applications that based on a Julian calendar. Consistently applying PGRs to creeping bentgrass will likely result in phytotoxicity that significantly reduces percent green cover, but ultimately expresses a much darker green color.