Trinexapac-ethyl, Propiconazole, Iron, and Biostimulant Effects on Shaded Creeping Bentgrass

Abstract

Creeping bentgrass (Agrostis stolonifera) is used extensively on temperate zone golf course greens, tees, and fairways, but often performs poorly in shade. Previous research has indicated that sequential applications of gibberellic acid (GA) inhibiting plant growth regulators (PGRs) such as trinexapac-ethyl (TE) increase cool-season turfgrass performance in 70-90% shade. This research was conducted to: 1) confirm appropriate TE application rates and frequencies for maintaining `Penncross' creeping bentgrass in dense shade in the mid-Atlantic region of the U.S.; 2) determine the efficacy of other PGRs, biostimulants, and iron (Fe); and 3) assess whether the addition of a biostimulant with TE would have additive, synergistic, or negative effects. The other compounds tested against TE and the control were: propiconazole (PPC), iron sulfate, CPR (a seaweed and iron containing biostimulant), and a generic seaweed extract (SWE) (Ascophyllum nodosum) plus humic acid (HA) combination. These treatments were applied to 88% shaded bentgrass every 14 days from May or June through October in 2001 and 2002, with turf quality, leaf color, root strength, photochemical efficiency, and antioxidant enzyme superoxide dismutase (SOD) activity being determined. While the quality of control plots fell below a commercially acceptable level by the second month of the trial, repeated foliar TE application provided 33% to 44% better quality throughout the experiment. Propiconazole resulted in 13% to 17% better quality through September of each year. Trinexapac-ethyl and PPC resulted in darker leaf color and increased mid-trial root strength by 27% and 29%, respectively. Canopy photochemical efficiency and leaf SOD activity were also increased due to TE in August of both years. Treatment with Fe, CPR, or SWE+HA did not have an effect on quality, root strength, SOD, or photochemical efficiency, but periodic increases in color were observed. The addition of CPR to TE in 2002 provided results that were not different from those of TE-alone. This and previous studies indicate that restricting leaf elongation with anti-GA PGRs is of primary importance for improving shade tolerance, while treatments that increase leaf color or chlorophyll levels without restricting leaf elongation are relatively ineffective.