Combining an Organic Fungicide Spray Program with a Warning System to Manage Sooty Blotch and Flyspeck on Apples

Sooty blotch and flyspeck (SBFS) is a fungal disease caused by melanized ectophytic fungi that colonize the waxy surface of apple fruit, reducing marketability and storability. In humid regions, SBFS is typically managed with calendar-timed applications of synthetic fungicides. However, this approach can be inefficient because it can over- or underestimate disease risk. Since SBFS risk is associated with periods of high relative humidity (RH), a weather-based warning system was developed in Iowa, USA. The system delays the second cover spray until 385 hours of RH ≥90% have accumulated since the first cover spray. Although this system can reduce fungicide use in conventional systems, it has not been evaluated with organic fungicides. To address this gap, field trials were conducted in Iowa in 2021 and 2022 on scab-resistant apple cultivars 'Redfree', 'Liberty', and 'GoldRush'. Organic fungicides were applied using a calendar-based schedule or three different RH-based thresholds of the warning system. Both growing seasons were unusually dry, limiting disease pressure. Results showed that the 385-h threshold reduced fungicide use by four sprays annually compared to the calendar-based approach, with no significant differences among treatments in SBFS severity or marketable yield. Disease incidence and yield varied by canopy position and harvest date, with GoldRush and lower-canopy fruit showing higher SBFS incidence. This study is the first in the USA to evaluate an SBFS warning system using organic fungicides. While additional validation is needed, findings suggest the system can reduce fungicide inputs in organic apple production without compromising fruit quality or yield.