Effects of plant growth regulators and fungicides on Rhizoctonia blight of tall fescue

In field experiments conducted in 1996 and 1997, the plant growth regulators (PGRs) flurprimidol, paclobutrazol and trinexapacethyl were applied individually to plots of tall fescue, Festuca arundinacea Schreb., cv. Rebel II at rates of 0.8, 0.6 and 0.3 kg a.i. ha-1. Twenty-four hours later, the fungicides propiconazole, iprodione, chlorothalonil, flutolanil and azoxystrobin were applied individually to the PGR-treated plots at rates of 1.7, 3.0, 6.0, 6.1, and 0.8 kg a.i. ha-1, respectively. Nontreated plots and plots treated with the PGRs or fungicides alone served as controls. Four hours after the fungicide applications, the turfgrass was inoculated with Rhizoctonia solani AG1IA, the cause of Rhizoctonia blight, and disease severity was assessed visually from 5 July to 2 August 1996, and from 27 June to 23 July 1997. In 1996, the PGR treatments had no significant effect on the severity of Rhizoctonia blight when compared to nontreated plots. Results were similar in 1997 with the exception that disease was significantly suppressed (P < 0.05) in plots treated with paclobutrazol 15 and 22 days after treatment. In both years, the propiconazole, flutolanil and azoxystrobin treatments significantly suppressed Rhizoctonia blight (P < 0.05) for at least 28 days when compared to a nontreated check, while iprodione and chlorothalonil resulted in significant disease suppression for at least 21 days. At the rates tested, flutolanil and azoxystrobin were the only fungicides tested that limited disease severity to < 10% for more that 28 days in each year of the study. Severity of Rhizoctonia blight was significantly greater (P < 0.05) on at least one assessment date in each year in plots treated with trinexapacethyl+propiconazole or with any of the PGRs tested +iprodione compared to plots treated with the fungicides alone. However, with the exception of a significant increase in disease in plots treated with paclobutrazol+chlorothalonil late in the 1996 epidemic, the remaining PGR+fungicide treatments had no significant effects on disease severity when compared to the effects of the fungicides alone.