Annual Bluegrass Response to Potassium and Calcium Fertilization and Soil pH.

Potassium fertilization has been shown to improve abiotic stress tolerance in turfgrass; however, its effect on turfgrass diseases has been inconsistent and not well understood.  A 3-yr field study was initiated in 2012 to determine the impact of K fertilization rate and K source on anthracnose disease of annual bluegrass turf.  Treatments were arranged as a 2 x 3 factorial with four replications.  Potassium chloride (KCl) and potassium sulfate (K₂SO₄) were applied at rates of 54, 109, and 218 kg K ha^-1 yr^-1.  Potassium nitrate (KNO₃) and potassium carbonate (K₂CO₃) were also included at the 218 kg K ha^-1 yr^-1 rate, as well as an untreated check.   All K treatments reduced disease severity compared to the untreated check (no K), regardless of K rate or K source; however, KCl applied at 218 kg K ha^-1 yr^-1 was slightly less effective at reducing disease severity than all other sources applied at the same rate during August 2013 and throughout 2014.  Critical mat K and leaf K concentrations needed to reduce anthracnose severity were calculated using nonlinear regression models (Cate-Nelson, linear plateau, and quadratic plateau).  All regression models provided an acceptable fit and predicted critical values for mat K between 43 to 70 mg K kg^-1 (Mehlich 3).  Only the Cate-Nelson and linear plateau models provided an adequate fit and critical value for leaf K with respect to anthracnose severity (AUDPC); critical leaf K values ranged from 17.3 to 28.6 g kg^-1.  The potassium fertilization effect on turf quality was similar to the disease severity response, with K fertilization improving turf quality.  Turf color, NDVI, and chlorophyll index were either not affected by K or only minor improvements were observed from K fertilization.