Fungicide efficacy and duration of protection against coffee leaf rust (Hemileia vastatrix) on commercial coffee farms in Hawaii

Coffee leaf rust (CLR, Hemileia vastatrix) is a recent arrival to Hawaii and has quickly become the most significant challenge to the State's small but economically important coffee industry. Frequent sprays of fungicides and the improvement of agronomic practices have been the primary management strategies for controlling this devastating disease. However, the high cost of products and labor coupled with the harmful effects of chemical inputs have made frequent applications of fungicides economically and environmentally unfeasible. The aim of this study was to determine fungicide efficacy against CLR, duration of protection, and cost of applications in field trials conducted on commercial coffee farms on Hawaii Island in 2022 and 2023. The translaminar fungicide was the most effective product tested, keeping CLR below 2 % over a 12-week period. Copper-based fungicides were also effective at keeping CLR below the 5 % action threshold, although the duration of protection was shorter (6–8 weeks). Copper fungicides were the most cost-effective of all products examined ($126/acre), followed by the translaminar ($140/acre). In contrast, the Bacillus-based fungicides and the botanical fungicide did not reduce CLR incidence and were more expensive to apply ($138-$198/acre). The findings presented here can be used by coffee growers to decide which fungicides to spray based on disease levels, time of year, management style, and budget. Future research is needed to determine optimal fungicide rotation strategies given the farm type and starting CLR incidence. This rotation will need to consider application timing, duration of product efficacy, as well as product compatibility.