Curative fungicide scheduling and bentgrass disease resistance affects dollar spot control

Bentgrass (Agrostis spp.) cultivars vary in resistance to dollar spot caused by Clarireedia jacksonii. Using a damage threshold to schedule fungicide applications has potential for reducing fungicide inputs. Two field trials managed as fairway turf in North Brunswick, NJ, from 2018 to 2021 assessed the effectiveness of damage threshold fungicide schedules to control dollar spot on bentgrass cultivars varying in disease resistance. Two factorially arranged randomized complete block designs (3 × 6 and 3 × 9, non-inoculated and inoculated, respectively) were used. A fungicide scheduling factor included a calendar schedule and two damage threshold schedules that applied fungicides either within 24 h or the next application day (NAD) once a 105 mm² m⁻² of symptomatic area was observed. The cultivar factor included six (Trial 1) and nine (Trial 2) bentgrass entries ranging from low to high dollar spot resistance. Damage threshold schedules on the resistant cultivar Declaration reduced fungicide inputs up to 78% compared to the calendar schedule. Reduced fungicide inputs for each cultivar were achieved by delaying disease onset in the non-inoculated trial and extending the application interval in both trials. The 24-h threshold schedule controlled dollar spot equivalent to the calendar schedule on more resistant cultivars. There was a lower risk of severe disease outbreaks when threshold applications were applied on the NAD schedule compared to the calendar schedule on the resistant cultivars. Thus, using a low damage threshold to apply fungicides on resistant bentgrass can effectively control dollar spot with fewer fungicide inputs.