Irrigation Following A Fall Proxy (Ethephon) Application Affects Spring Seedhead Suppression Of Meyer Zoysiagrass

‘Meyer’ zoysiagrass (Zoysia japonica Steud.) is one of the most common zoysiagrass cultivars used on golf course fairways in the transitional climatic zone. Meyer zoysiagrass provides an excellent turf surface for golf course fairways because of its high turf quality, durability, low N fertility requirements, and tolerance of abiotic stresses (Patton, Schwartz, & Kenworthy, 2017). However, spring seedhead (inflorescence) production causes a decline in turf aesthetics and an increase in maintenance costs (Brosnan, Breeden, Elmore, Patton, & Weisenberger, 2012; Patton, Schortgen, Hoyle, Harrell, & Reicher, 2018). A recent study demonstrated that a fall application of Proxy [ethephon (2-chloroethylphosphonic acid); Bayer Environmental Science] plant growth regulator can be an effective option to suppress spring seedhead production in Meyer zoysiagrass (Patton et al., 2018). This fall timing is within the same application window recommended for preventative fungicide applications of Rhizoctonia large patch disease (RLPD) [Rhizoctonia solani Kuhn Anastomosis Group (AG)-2-2 LP], which is the primary disease affecting zoysiagrass (Patton et al., 2018). Benelli et al. (2018) reported reduced RLPD severity when fungicides were applied to the lower plant canopy of zoysiagrass, such as the leaf sheath or stem, compared with applications to the leaf blade. Thus RLPD control may be improved by either higher spray application volumes or possibly postapplication rainfall or irrigation to allow for greater canopy penetration. Golf course superintendents could reduce labor by tank-mixing the fall fungicide application for large patch with a Proxy application for spring seedhead suppression; however, we are unsure how postapplication irrigation influences the efficacy of Proxy. The objective of this experiment was to determine the effect of postapplication irrigation volumes on fall-applied Proxy for spring suppression of Meyer zoysiagrass seedheads. Experiments were initiated on mature Meyer zoysiagrass on 3 Oct. 2017 at the W.H. Daniel Turfgrass Research and Diagnostic Center in West Lafayette, IN, and on 18 Oct. 2017 at the Rocky Ford Turfgrass Research Center in Manhattan, KS. Research plots measured 25 ft2 in Indiana and 50 ft2 in Kansas and were maintained at golf course fairway heights of 0.5 or 0.625 inches, respectively. A one-way treatment structure containing seven treatments was arranged in a randomized complete block design with four replications at each location. The six treatments included Proxy (ethephon), plant growth regulator applied at 5 fl oz 1000 ft–2 with six postapplication irrigation volumes, plus an untreated control for comparison (Table 1). Proxy was applied with a CO2-pressurized boom sprayer equipped with XR80015VS flat-fan nozzles calibrated to deliver 87 gal acre–1 at 28 psi in Indiana and with XR8003VS flat-fan nozzles calibrated to deliver 43 gal acre–1 at 30 lb inch–2 in Kansas. Each Proxy application was allowed to dry for 30 to 60 min before calculated irrigation volumes for each treatment were applied by hand with a hose-end wand with a flow meter to deliver postapplication irrigation treatments of 0, 0.0625, 0.125, 0.25, 0.5, and 1.0 inches of water. Tebuconazole [1-(4-chlorophenyl)-4,4-dimethyl-3-(1,2,4-triazol-1-ylmethyl)pentan-3-ol] at 0.7 lb a.i. acre–1 was applied separately, 1 to 2 wk prior, to both experimental areas for control of RLPD. Where Proxy0 is the percentage of seedhead coverage of Proxy with no irrigation and Proxyx is the percentage of seedhead coverage of Proxy with x inches of irrigation. Location × treatment interactions were significant; therefore, data for each location were analyzed separately. All data were subjected to ANOVA via the GLIMMIX procedure (SAS 9.4; SAS Institute Inc., Cary, NC), with block as a random effect. Residual normality was tested via the UNIVARIATE procedure in SAS. Linear regression analysis between seedhead counts and irrigation volume was performed via the REG procedure in SAS. Means were separated by Fisher's Protected LSD test (α = .05) when F-tests were significant (P ≤ .05). Consistent with a previous report by Patton et al. (2018), fall Proxy application with no irrigation afterwards reduced spring seedheads by up to 45% compared with the untreated (Table 1). At both locations, irrigating after a Proxy application influenced the cover and quantity of seedheads, where 1.0 inches of irrigation increased seedhead coverage compared with no postapplication irrigation. Furthermore, plots receiving 1.0 inches of postapplication irrigation had similar seedhead production to the untreated plots at both locations (Table 1). The effects of irrigation were obvious in Indiana, where the spring seedhead coverage of Proxy treatments receiving any postirrigation had seedhead cover similar to the untreated control. The results were less consistent in Kansas, with plots receiving only the 1.0-inch postirrigation treatment having increased seedhead cover compared with the no-irrigation treatment (Table 1). Zoysiagrass seedhead production across the transition zone is often variable from year to year, as well as between locations within years for reasons that are not entirely understood. The results for seedhead cover and counts in Kansas indicate that similar seedhead suppression was achieved with Proxy applications that received 0 to 0.5 inches of irrigation (Table 1). Greater variability in seedhead count data in Indiana led to no statistical differences in seedhead count among treatments. Linear regression results between seedhead counts and irrigation volume indicated a reduction in Proxy efficacy from increasing amounts of postapplication irrigation (Indiana: P ≤ .0238; Kansas: P ≤ .0002) but did not explain the majority of the relationship between seedhead production and irrigation volume following Proxy application (Indiana: R2 = .2115; Kansas: R2 = .4683) (Figure 1). Ethephon absorption and transport primarily occurs through foliar uptake (McCollough & Sidhu, 2014). As such, increased irrigation volumes in this experiment probably washed Proxy off the zoysiagrass leaves, causing the observed reduction in efficacy. No zoysiagrass injury occurred as a result of the treatments in this experiment and there were no visual turf quality differences among the Proxy treatments at either location (data not shown). The incidence and severity of RLPD in the fall and spring in response to irrigation volume was not evaluated at either site; however, as mentioned by Benelli et al. (2018), further investigation is required on the effectiveness of postapplication irrigation volume on large patch control. Although they were not entirely consistent across locations, our results suggest reduced Proxy's efficacy when followed by increasing irrigation volumes to water in fungicide applications, especially with 1.0 inches of irrigation. Our data, combined with that of Benelli et al. (2018) suggest that the most effective control of both zoysiagrass seedheads and RLPD would result with no irrigation after Proxy application and application should be made at higher spray volumes of 2 gal 1000 ft–2. Furthermore, ethephon and RLPD fungicide applications should be made when the forecast does not predict significant precipitation. Authors Patton, Braun, Schortgen, and Hoyle declare that they have no conflict of interest. Author Reicher is employed by Bayer Environmental Science who produce Proxy (ethephon). He was involved in the design of the experiment and provided comments to the manuscript but was not responsible for field research, data collection, or data analysis. The authors express their gratitude to Bayer CropScience LP for funding this research. This work was supported by Hatch project 1004762 from the USDA National Institute of Food and Agriculture.