Potassium Source and Rate Effect On Anthracnose Severity of Annual Bluegrass.

Potassium (K) is the second most abundantly used element by plants and has been shown to increase stress tolerance (heat, drought, and wear) in cool season grasses.  Previous research on anthracnose (caused by Colletotrichum cereale Manns) disease has shown that potassium nitrate fertilization reduced disease severity compared to other nitrogen (N) sources.  However, the effect of potassium fertilization on anthracnose severity is unclear.  The objective of this field study was to determine the effect of K source and rate on anthracnose severity of annual bluegrass [Poa annua L. f. reptans (Hausskn) T. Koyama] putting green turf.  This study was designed as a randomized complete block with four replications.  Two K sources, potassium sulfate (K₂SO₄) and potassium chloride (KCl), were applied at N:K elemental ratios of 1:1, 2:1, and 4:1. Potassium carbonate (K₂CO₃) and potassium nitrate (KNO₃) applied at a N:K ratio of 1:1 were also included.  A no-K control was included in the trial as well as a no-N control with potassium chloride applied at the same K rate as the 1:1 treatments.  All treatments were applied biweekly along with 4.9 kg ha^-1 of N as urea (excluding the no-N control) beginning 24 and 23 Apr. through 10 and 1 Nov. in 2012 and 2013, respectively.  In both years, treatments that received both N and K had significantly less anthracnose severity then either nutrient alone, regardless of K source and rate.  In 2012, few differences were observed between potassium sources and rates; however, by 6 Aug. 2013, greater rates of potassium (1:1 and 2:1) had significantly less disease than the lowest potassium rate (4:1).  Interestingly, K fertilization alone (no N) had significantly less disease then N fertilization alone (no K) by the end of 2012 and throughout most of 2013. Soil and leaf tissue samples were also collected and will be used to determine sufficiency levels relative to anthracnose severity.