Foliar N Concentration and Reflectance Meters to Guide N Fertilization for Anthracnose Management of Annual Bluegrass Putting Green Turf

Abstract

Site‐specific management using objective assays to determine N requirement based on canopy reflectance could improve current recommendations for cultural control of anthracnose (Colletotrichum cereale Manns sensu lato Crouch, Clarke, and Hillman). The objectives of this study were to determine the relationships among foliar sap nitrate–nitrogen (NO3–N) concentrations, canopy reflectance indices (normalized difference vegetation index [NDVI] and chlorophyll index [CI]), foliar total N concentrations, and N rate on anthracnose severity in annual bluegrass (ABG), Poa annua L. f. reptans (Hausskn) T. Koyama. A 2‐yr field study was conducted on ABG putting green turf in Storrs, CT. Nitrogen treatments were applied as urea at 0.0 to 36.8 kg ha−1 every 14 d in spring and summer of 2011 to 2012. Anthracnose severity declined linearly with increasing biweekly N rate from 0 kg ha−1 up to the estimated critical level of 12.8 kg ha−1. A minimum critical foliar N concentration of 33.5 g kg−1 maintained anthracnose at ≤32% plot area. Canopy reflectance indices were correlated with foliar N concentration. Anthracnose severity decreased linearly as relative NDVI and relative CI increased. Maintaining relative NDVI between 0.94 to 0.98 through N fertility alone would provide a ∼76 to 93% probability of maintaining ≤32% turf area infested, or relative CI range of 0.73 to 0.93 had the same result ∼40 to 98% of the time.