Summer patch disease severity on Kentucky bluegrass in response to fertilizer source

Summer patch is caused by the ectotrophic, root-infecting fungus Magnaporthe poae Landschoot and Jackson. The disease, which often infects high maintenance turf, can be difficult to control because root infection often occurs six to eight weeks before the appearance of foliar symptoms. Disease severity is reduced when turf is fertilized with ammonium nitrogen (N) sources, compared to nitrate or urea sources of N. Thiosulfate, from (NH₄)₂S₂O₃ or K₂S₂O₃, is a nitrification inhibitor which may enhance ammonium uptake of turf by slowing nitrification. N-SURE is a triazone-based, slow release N source that is commonly used to fertilize turfgrass. Field studies were conducted from 1995 to 1996 on Kentucky bluegrass (Poa pratensis L.) grown on a Nixon loam (fine-loamy, mixed, mesic Typic Hapludult) to evaluate the effectiveness of the several N and K sources for their ability to control summer patch disease. Nitrogen fertilizers, N-SURE, (NH₄)₂S₂O₃, and (NH₄)₂SO₄, were applied in combination with either K₂SO₄ or K₂S₂O₃. The severity of summer patch was greater when the turf was fertilized with N-SURE in 1995 and 1996 and urea in 1996 compared to (NH₄)₂S₂O₃. The N sources, (NH₄)₂SO₄ and (NH₄)₂S₂O₃, were strongly acidifying to the upper 10 cm of soil and were very effective in controlling summer patch. The application of K₂S₂O₃ slightly acidified the upper 5 cm of soil but did not suppress the development of summer patch. The ability of thiosulfate to act as a nitrification inhibitor did not appear to play a role in the suppression of summer patch. Since foliar bum may occur if (NH₄)₂SO₄, (NH₄)₂S₂O₃, or K₂S₂O₃ are applied to turf without irrigation, the application of water after their use is recommended.