Take‐all root rot response to nitrogen rate and source in ultradwarf bermudagrass

Abstract Take‐all root rot (TARR) is a problematic disease of ultradwarf bermudagrass ( Cynodon dactylon × Cynodon transvaalensis ) putting greens. The etiology and chemical management of TARR are well characterized, yet little is known about how fertilization management, including nitrogen (N) rates and sources, affects TARR severity on ultradwarf putting greens. To address the lack of knowledge, two separate greenhouse experiments were conducted. In the first of this two‐part greenhouse study, ultradwarf bermudagrass cores were inoculated with Gaeumannomyces graminis and fertilized with ammonium sulfate at rates of 0, 6.1, 18.3, 24.4, 30.5, 42.7, and 48.8 kg N ha −1 week −1 for 8 weeks to investigate inoculated turfgrass performance at different N rates. In the second part of the experiment, four distinct N sources—ammonium sulfate, potassium nitrate, calcium nitrate, and urea—were applied at a rate of 24.4 kg ha −1 week −1 for 8 weeks to investigate inoculated turfgrass performance to different N sources. The turfgrass was evaluated for percent root necrosis and dry root weights. This greenhouse study indicated that N rates at or greater than 18.3 kg ha −1 week −1 would reduce TARR severity on ultradwarf bermudagrass compared to turfgrass that received the lowest N rate at 6.1 kg ha −1 week −1 and the non‐fertilized control. Additionally, turfgrass that received ammonium sulfate as an N source had significantly less necrosis than all other N sources, and urea and the non‐fertilized control had the highest, similar necrosis rates.