Evaluation of Nitrogen Source on the in Vitro Growth and Morphology of Rhizoctonia Solani AG 2-2 LP.

Large patch 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 found LP isolates grown on ammonium-based media remained white, while isolates grown on nitrate-based media and potato dextrose agar (PDA) melanized and turned brown.  This study aimed to evaluate the effects of nitrogen source and a known pH buffer on R. solani AG 2-2 LP radial growth and morphology.  Thirty-one isolates were initially grown on antibiotic amended PDA and transferred to water agar (WA).  Mycelial plugs from WA were transferred to basal media amended with ammonium sulfate (AMS), calcium nitrate (CN), 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) buffer.  Radial growth was recorded after four days at 25°C.  Isolates grown on CN had the greatest radial growth, isolates grown on AMS were intermediate, and urea-grown isolates had the least radial growth.  Radial growth on CN media increased as concentration increased, while higher urea concentrations had less radial growth than lower concentrations of urea.  There was no difference in radial growth between AMS concentrations.  Fumaric acid increased radial growth on all N-sources.  All isolates melanized on media amended with CN and urea. Isolates grown on 200 µg ml^-1 AMS melanized with FA, but remained white without FA.  Mycelial density in liquid media will be evaluated in future experiments.  If melanin plays a role in the infection process, a fertility program involving AMS may be useful in an integrated strategy for large patch control.