Development of Novel Strategies for Control of Spring Dead Spot in Bermudagrass

Abstract

Spring dead spot (SDS) is one of the most severe diseases of bermudagrass in Australia, New Zealand, and in the United States where bermudagrass goes into winter dormancy. Several key factors have hampered research on SDS in the past and are likely due to the lack of complete knowledge about the etiology and epidemiology of SDS, lack of knowledge for optimal fungicide efficacy, and the erratic distribution of SDS in the field, which contributes to experimental error. The objectives of this research were to optimize application methods, application timings, and fungicide efficacy, to evaluate rating methods for analysis of SDS incidence, and to identify the causal organism for SDS of bermudagrass in North Carolina. Between 2002 and 2004, we evaluated the efficacy of five application methods, twelve application timings, and four fungicides. No significant differences were detected among application methods; however application in higher volumes of water tended to provide better control. No significant differences were noted among applications timings when compared to the untreated control, but multiple applications starting in August or September tended to provide better control than single or late season applications. Spring applications had no significant effect on SDS incidence or recovery rate. Of the fungicides that were evaluated in this project, fenarimol and propiconazole were most effective, providing from 44 to 89% and 42 to 54% control, respectively. In the spring of 2003, comparison of three different assessment methods for SDS were evaluated: digital photography (DP), visual estimation (VE), and the point-intersect method (PI). Results of this study indicate that DP is more effective than VE for assessment of SDS incidence. Digital photography consistently produced higher r2 values and lower CV, MSD, and MSE values than VE. Reductions in experimental error translated directly to differences in mean separations used to compare disease incidence in response to fungicide treatments. Digital photography was not consistently more accurate or precise than PI. In total, two-hundred twenty-one isolates of Ophiosphaerella were collected between 2003 and 2004. Of the isolates collected, Ophiosphaerella korrae (186 of 221) was the predominant organism isolated from SDS symptomatic bermudagrass. Ophiosphaerella herpotricha (22 of 221) was detected at low levels in the Charlotte, Raleigh, and Wilmington regions, whereas O. narmari was never detected at all in this study.