Potassium Nutrition Affects Anthracnose on Annual Bluegrass

Core Ideas This study provides evidence that K deficiency can increase anthracnose severity of annual bluegrass turf. Deficiency can be identified with testing of mat layer and tissue K concentrations. Potassium values less than 43 mg kg −1 in the mat layer (Mehlich 3 extractant) and 20 g kg −1 in leaf tissue will increase anthracnose severity of annual bluegrass.Potassium fertilization can improve stress tolerances in turfgrass; however, its effect on turfgrass diseases is inconsistent and not well understood. A 3‐yr field study was initiated to determine the effect of K fertilization rate and K source on anthracnose disease of annual bluegrass turf. Potassium chloride and K 2 SO 4 were applied at K rates of 54, 109, and 218 kg ha −1 yr −1 as a 2 × 3 factorially arranged randomized complete block design with four replications. Potassium nitrate and K 2 CO 3 were also included at the 218 kg K ha −1 yr −1 rate, as well as an untreated check. Nonlinear regression models were used to estimate critical concentrations of K in the mat layer and tissue with respect to anthracnose severity. All K treatments reduced disease severity compared to the untreated check (no K); however, KCl was less effective than the other sources when applied at the 218 kg K ha −1 yr −1 rate. The Cate–Nelson regression model predicted that K values less than 43 mg kg −1 in the mat layer (Mehlich 3 extractant) and 20 g kg −1 in leaf tissue will increase anthracnose severity of annual bluegrass (ABG). This study provides evidence that K fertilization can reduce anthracnose severity and that the severity of this disease can be correlated with mat and tissue K concentrations. Thus, it is important to monitor K in ABG turf and to maintain sufficient levels to reduce disease severity and improve turfgrass quality.