Bentgrass Disease resistance affects fungicide scheduling and dollar spot control
Creators: name: Zhang, Pingyuan
Publisher: name: No Publisher Supplied
Year: 2024

description: Dollar spot caused by Clarireedia spp. is a wide-spread fungal disease of most cool- and warm-season turfgrasses. Bentgrass (Agrostis spp.) is a popular choice for high-quality playing surfaces on golf courses. Bentgrass species and cultivars vary in resistance to dollar spot. Five field trials managed as fairway turf were conducted from 2018 to 2023 to assess the impact of bentgrass disease resistance on fungicide scheduling strategies to control dollar spot. Curative fungicide scheduling has the greatest potential for reducing fungicide inputs but also the greatest risk of unacceptable disease outbreaks. Two field trials were conducted to assess the effectiveness of using damage-threshold fungicide scheduling to control dollar spot on bentgrasses with a range of disease resistance. Factorially arranged randomized complete block designs were used to evaluate bentgrass (species and cultivars) and three fungicide schedules, which included a calendar schedule and two damage-threshold schedules sprayed at 24-h or the next spray-day (NSD) after the damage-threshold was observed. The damage-threshold schedules reduced fungicide inputs up to 78% compared to the calendar schedule with the greatest reduction observed on cultivars with greater resistance to dollar spot. The 24-h damage-threshold schedule achieved acceptable control of dollar spot on all cultivars in both trials; however, acceptable control of dollar spot with the NSD schedule was only feasible on disease resistant cultivars. Unacceptable control of dollar spot resulting from delaying threshold sprays to the NSD on susceptible cultivars typically occurred when disease pressure was high. A model-based fungicide control strategy also has the potential for improving the accuracy of fungicide scheduling and reducing fungicide use. A third field trial evaluated the feasibility of using novel action thresholds from a logistic regression model to schedule fungicide applications to control dollar spot on disease resistant (Declaration) and susceptible (Independence) creeping bentgrass cultivars. Twenty-three action thresholds from the Smith-Kerns dollar spot predictive model were used to schedule fungicide sprays, which included four risk index (RI) thresholds (20, 30, 40, and 50%); four assessments of the change in the RI over time (RI slope); and 15 combinations of RI thresholds and RI slope. The four assessments of RI slope considered a positive change in RI value over the previous 5-d, forecasted 5-d, previous or forecasted 5-d, or previous and forecasted 5-d as action thresholds to schedule fungicide applications. The trial also included calendar and curative damage-threshold schedules for comparisons. All model action threshold schedules maintained acceptable dollar spot control equivalent to the calendar schedule on the resistant cultivar Declaration, which resulted in reductions in fungicide input up to 67% compared to the calendar schedule. However, for the susceptible cultivar Independence, only the 20% and 30% RI action thresholds and the same RI thresholds combined with a positive RI slope over the previous or forecasted 5-d maintained acceptable control of disease; the 30% RI combined with RI slope reduced fungicide input 33% compared to the calendar schedule. Anecdotal observations, albeit inconsistent, suggest that fungicides applied the previous autumn can suppress the development of dollar spot the following spring. Two field trials evaluated the impact of autumn fungicide timing and chemistry on the onset and progress of dollar spot during the following growing season on creeping bentgrass cultivars varying in resistance to the disease. Additionally, the concentration of Clarireedia spp. was evaluated in the autumn after fungicide schedules ended, as well as the subsequent spring before or at the onset of disease. Both trials evaluated non-treated controls, seven calendar schedules sprayed with a tank mixture of fluazinam and propiconazole and one sprayed with chlorothalonil. There were three additional fungicide schedules in the second trial. The first trial was conducted on ‘007’ creeping bentgrass and the second trial evaluated ‘Coho’, ‘007’, and ‘Independence’ as disease resistance (cultivar) factor. The sequential application of the tank mixture in September and October was the fungicide level with the fewest applications and was among treatments with the greatest suppression of disease during the subsequent spring in both trials. In addition, sequential applications of chlorothalonil in September, October and November had little or no effect on AUDPC in both trials. Disease onset and AUDPC during the onset period were strongly influenced by the cultivar factor, which interacted with the fungicide factor in one year of the second trial. The delay in disease onset was the greatest, and disease severity was the least on Coho creeping bentgrass. The 20% risk index threshold schedule was the only treatment that provided the greatest delay in disease onset and suppression of AUDPC across all cultivars during the subsequent season in both years of the second trial. The most effective timings of the tank mixture applied on the disease resistant cultivar Coho delayed the onset of dollar spot at least seven weeks in the second trial. Disease severity and delay in disease onset were positively and negatively correlated, respectively, with the pathogen load measured in autumn and the subsequent spring in both trials. These results support the hypothesis that suppression of pathogen load in autumn carries forward the following year reducing dollar spot. It is apparent that disease resistant cultivars have great potential for reducing fungicide inputs. Further research is needed to evaluate other fungicide chemistries and whether properly timed sprays during autumn can reduce fungicide inputs on disease susceptible cultivars the subsequent year.Dollar spot resistant bentgrass cultivars provide flexibility when choosing fungicide scheduling strategies that maintain excellent disease control equivalent to the calendar schedule. Damage-threshold schedules provide the greatest potential for reducing fungicide input for resistance cultivars. However, using damage-threshold schedules on susceptible cultivars was not feasible because of a greater risk of unacceptable disease outbreaks. Substantial reductions in fungicide inputs on a dollar spot resistant creeping bentgrass cultivar can be realized when using higher risk index thresholds along with risk index slopes from the Smith-Kerns disease predictive model. End of the season pathogen load as influenced by cultivar, fungicide timing, and fungicide chemistry can have a significant impact on dollar spot development the subsequent year.