Potassium fertilization effects on dollar spot of annual bluegrass and creeping bentgrass

Abstract The effect of potassium on dollar spot of annual bluegrass (ABG; Poa annua L. forma reptans (Hausskn.) T. Koyama) and creeping bentgrass (CBG; Agrostis stolonifera L.) is poorly understood. Two field trials were conducted in 2020 and 2021 to determine the effect of K fertilization on dollar spot of ABG and CBG turf grown on a sand mat layer overlying a sandy loam (fine‐loamy, mixed, semiactive, and mesic Typic Hapludults) and mowed at 2.8 mm. A 4 × 2 factorial, randomized complete block design evaluated K (potassium sulfate) applied at 0, 3.4, 6.9, and 13.8 kg ha −1 every 2 weeks and N (urea) applied at 4.9 kg ha −1 every 7 or 28 days over 20 weeks. Infection centers were counted over a 2‐week period each year after inoculation with Clarireedia jacksonii in mid‐September and used to calculate the disease severity. Increasing K fertilization rate consistently increased dollar spot severity on ABG and CBG. Higher N rate either slightly increased or did not affect disease severity on ABG, and either decreased or had no effect on CBG. This is the first study to document the impact of K fertilization on dollar spot severity of ABG turf. Regression analysis indicated that increases in both leaf tissue and mat layer K were associated with greater dollar spot severity on both species. Future research should determine whether the increased dollar spot response to K fertilization occurs at higher antecedent mat layer and leaf tissue K. Additionally, a broader range of N rates may clarify the dollar spot response.