Impact of Nitrogen Source and a pH Buffer on the in Vitro Growth and Morphology of Rhizoctonia Solani AG 2-2 LP

Large patch (LP) disease of zoysiagrass (Zoysia japonica Steud.) caused by the fungal pathogen Rhizoctonia solani AG 2-2 LP is an important disease in the United States transition zone that limits the utility and aesthetics of lawns and golf fairways. A preliminary study has shown that R. solani grown on ammonium-based media remained white, while isolates grown on nitrate-based media and potato dextrose agar (PDA) were melanized and brown. This study aimed to evaluate the effects of nitrogen source and a pH buffer on the radial growth and morphology of R. solani AG 2-2 LP. Forty isolates were initially grown on antibiotic amended PDA and transferred to water agar. Mycelial plugs from water agar were transferred to basal media amended with ammonium sulfate (AMS), calcium nitrate, or urea at concentrations of 0, 50, 100, 200, 400, and 800 µg mL^-1. A second set of each concentration was also amended with fumaric acid (FA) pH buffer. All media was brought to pH of 7.0 prior to autoclaving. Radial growth was recorded after 4 d. Isolates grown on calcium nitrate had the greatest radial growth, isolates grown on AMS were intermediate, and urea-grown isolates had the least radial growth (P < 0.0001). Radial R. solani colony growth increased with increasing calcium nitrate concentration, but growth decreased with increasing AMS and urea concentrations. Fumaric acid increased radial growth on all N-sources. All isolates melanized on media amended with calcium nitrate and urea. Isolates grown on 200 µg mL^-1 AMS melanized with FA, but remained white without FA. If melanin plays a role in pathogen infection and pathogen survival, a fertility program involving AMS may be useful in an integrated strategy to suppress large patch.