Justice Ruwona, Ph.D. student and Dr. Lee Miller
Dollar spot is the most fungicide intensive and expensive disease to control on golf courses. New bentgrass cultivars with enhanced dollar spot resistance are available but adoption has been slow due to the expense of renovation procedures and unknown agronomic attributes of the new varieties. This research aims to evaluate interseeding techniques of bentgrass areas using dollar spot host resistance as the phenotypic marker to indicate a successful conversion with funding provided by the United States Golf Association Mike Davis Program for Advancing Golf Course Management.
Greenhouse Experiments
This study was conducted to determine an optimal ratio of resistant to susceptible cultivars in a blend that will result in dollar spot reduction. Seed blend ratios between susceptible and resistant cultivars serve as proxies to model the anticipated reduction in disease that a turf manager might expect upon transitioning to a more dollar spot-resistant cultivar. Greenhouse experiments were conducted on cultivar blend ratios (0%, 10%, 25%, 50%, 75%, 90%, 100%) of dollar spot resistant (‘Pure Select’, ‘007XL’, ‘Coho’) and susceptible (‘Penncross’) bentgrass inoculated with two different dollar spot isolates. Disease severity was assessed visually and through image analysis software. Results demonstrated that blending susceptible and resistant cultivars reduces dollar spot severity compared to using solely susceptible cultivar ‘Penncross’. Increasing resistant cultivar proportions led to a consistent decrease in dollar spot severity, with blends containing 90% resistant cultivar achieving up to 32% less disease. Based on these findings, there is no optimal ratio of susceptible to resistant cultivars. Instead turf management strategies should focus on maximizing the proportion of resistant cultivar phenotypes within the stand to achieve the greatest reduction in dollar spot.
Field Trials
In July 2023, three dollar spot resistant cultivars (007 XL, Pure Select, Coho) were slit seeded into a susceptible bentgrass cultivar under five herbicide/PGR treatments and a non-treated control to control Poa annua invasion. These field trials are being replicated at two other locations (Wisconsin and Penn State). Plots are being maintained at fairway height of 0.5 inches. Poa annua flowering was rated in spring 2024 as a clear indicator of presence. Poa annua was more than three times higher in glyphosate treated plots compared to Poacure treated plots, with Poacure demonstrating the most effective control of all herbicide/PGR treatments. In May 2024, plots were divided with one half left to natural disease development and the upper half receiving a curative fungicide (Secure – 0.5 fl oz/1000 sq ft) when a threshold of five dollar spot infection centers occurs. Disease severity ratings through July 31st shows interseeding without using herbicide treatment has more than 10% higher disease severity than plots treated with glyphosate. There were no significant disease severity differences among the three resistant cultivars. Due to the heavy disease pressure experienced thus far in the season, all treatments have received an average of three fungicide applications. We will continue to monitor disease activity throughout the season to determine if interseeding method related disease variations lead to reduced fungicide sprays in a cost benefit analysis. Grass clippings are being collected to track grass stand composition and will be assessed with molecular markers developed by colleagues at the USDA-ARS.