Evaluation of Nitrogen Source and pH On the Mycelial Growth of the Spring Dead Spot Pathogens Ophiosphaerella Herpotricha and O. Korrae in Culture.

Spring dead spot of bermudagrass caused by Ophiosphaerella korrae and O. herpotricha is a destructive disease lacking consistent control methods. Recent field studies indicate nitrogen source and/or soil pH may play significant roles in suppressing spring dead spot. Two in vitro mycelial growth assays were utilized to assess the impact of nitrogen source and pH on the spring dead spot pathogens O. korrae and O. herpotricha. To assess pH, potato dextrose agar amended with antibiotics was adjusted to pH ranges 3-9 with sodium hydroxide and lactic acid. Calcium nitrate or ammonium sulfate was amended at concentrations of 0-800 ppm to a basal growth medium to investigate nitrogen source impact. Fifty μl of a mycelial homogenate containing O. korrae or O. herpotricha was pipetted into a 5mm well excised from the middle of each amended plate.  Radial mycelial expansion was measured after 8 and 12 days of growth on the pH and nitrogen amended media, respectively. Analysis of variance was conducted and means separation was done with Tukey’s test (p=0.05). Among all treatments O. korrae grew significantly more in the amended media than O. herpotricha. Growth of O. herpotricha was not statistically different regardless of nitrogen source, however O. korrae grew significantly more through media amended with CaNO_3. Concentrations of 400 and 800 ppm of both nitrogen sources inhibited growth of both species. In the pH experiment, growth of O. korrae and O. herpotricha was diminished on pHs 3 and 4, but was not statistically different among pHs 5-9. Future experiments will include mycelial weight as a response variable since mycelium density appeared to be affected by the nitrogen source treatments. At this point, results from this in vitro experiment do not correlate with field studies demonstrating spring dead spot suppression with the two nitrogen sources.