Evaluation of Nitrogen Source, Sulfur, and Fungicides for Suppression of Spring Dead Spot of Bermudagrass.

Prior field research suggests the use of particular nitrogen sources, soil pH manipulation, and fungicides may be effective in suppressing spring dead spot (SDS) of bermudagrass.  A three-year field study on a plot naturally infested with SDS caused by Ophiosphaerella herpotricha was conducted utilizing a strip-plot design with nitrogen (N) source as the main plot and sulfur and fungicide as subplots. N source treatments included urea, ammonium sulfate (AMS), calcium nitrate (CN), Uflexx, and Umaxx applied at 49 kg·ha^-1 in June, July and August. Sulfur (98 kg·ha^-1) was applied to plots at the same time as N, or left untreated. Plots were treated with tebuconazole (0.82 kg·ha^-1) once or twice during the fall or left untreated. Disease severity was assessed using digital imaging analysis.  An in vitro study was also conducted to assess the impact of N source and pH directly on the mycelial growth of the SDS pathogens O. herpotricha and O. korrae.  To assess N source, isolates were placed on a modified Melin-Norkrans media amended with 0-800 ppm of either AMS or CN.  For pH assessment, isolates were placed on potato dextrose media adjusted to pH’s 3-9 using sodium hydroxide or lactic acid. All data were analyzed using the PROC MIXED procedure and pair-wise comparisons were generated using the LSmeans command. In field research, SDS severity was not significantly different among nitrogen source treatments. SDS was less severe the following spring in plots receiving sulfur and/or tebuconazole, when compared to plots receiving only N. Fungicide plus sulfur treatments provided more SDS suppression than fungicide alone. Growth of both SDS species was significantly lower on AMS amended media than on CN amended media. For both species, no mycelial growth was recorded at pH 3, and was significantly lower on pH 4 compared to pHs 5-9.