The Impact Nitrogen Application Rates, Nitrogen Source and Ambient pH Have on Sclerotinia homoeocarpa on Agrostis stolonifera Putting Greens.

Dollar spot is a disease which infects many turfgrass species and has been associated with nitrogen deficient turfgrass. The pathogen Sclerotinia homoeocarpa which causes the disease, is controlled predominately by applying fungicides. Fungicides controlling dollar spot have been questioned as to the potential risk they may pose to the environment. It is known that pH plays a role in oxalic acid production by the pathogen and potentially affecting the pH of the foliar tissue and soil by using different nitrogen sources which could reduce dollar spot severity. The objective of this trial was to evaluate the impact of nitrogen source and application rate on dollar spot severity. The field trial was run over a two year period on two putting greens, located in Verona, Wisconsin and Glenview, Illinois. Field parameters evaluated included percent infected area per plot, number of infection sites per plot, foliar pH, soil pH, turfgrass quality and percent foliar tissue nitrogen content. Lab parameters evaluated included oxalic acid production, fungal biomass accumulation, potato dextrose broth (PDB) pH change, and different initial ambient pH. The results from the field trial showed that only nitrogen application rates effected dollar spot severity while nitrogen source did not. Results from the lab trial showed that oxalic acid was produced in significantly greater quantities (P=0.05) when initial pH was neutral compared to acidic. We conclude that nitrogen source does not play a role in dollar spot severity, and when nitrogen application rates increase there is a reduction in dollar spot severity. We conclude that initial ambient pH determines the amount of oxalic acid produced by the pathogen and when more oxalic acid is produced by the pathogen ambient pH becomes more acidic.