Plant health effects of fungicides alone and in combination with plant growth regulator on improving summer turf performance in annual bluegrass

Abstract Annual bluegrass ( Poa annua L.) is sensitive to high‐temperature stress, and approaches that can improve plant growth during summer months are important for golf courses managing P. annua putting greens. The objective of this 2‐year field trial was to determine plant health benefits for selected fungicides and the combination with a plant growth regulator (PGR), trinexapac‐ethyl (TE) on P. annua growth under putting green conditions during summer months. The following treatments were foliar sprayed at 14‐day intervals from June to September in 2020 and 2021: (1) untreated control with water, (2) Daconil Action, (3) Appear II, (4) Daconil Action and Appear II, and (5) Daconil Action, Appear II, and Primo Maxx (TE). Applying individual and combination treatments resulted in significant improvements on P. annua summer performance, as manifested by increased visual turf quality and other vegetation indices evaluated using multispectral radiometer (normalized difference vegetation index, leaf area index, and stress index or digital camera [percent canopy cover and dark green color index]) in both years. The combined treatment programs, Daconil Action and Appear II or Daconil Action, Appear II, and Primo Maxx were more effective than the untreated control and each individual treatment. The results suggest that there existed synergistic effects of multiple fungicides and PGR, which could be particularly useful in promoting plant health of P. annua under heat stress conditions.