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

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 (NO 3 –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.