Comparative evaluation of fungicide effectiveness in controlling coffee leaf rust in Hawaii

Coffee leaf rust (CLR), Hemileia vastatrix, is a disease that has severely impacted the major coffee growing areas of the world. The discovery of CLR in Hawaii in October 2020 was devastating news to local growers, as it quickly spread to all coffee-growing areas, threatening the second-highest-valued crop in the State. Although the fungicide Priaxor Xemium had been approved as a Section 18 emergency exemption, there was still an urgent need for true systemic fungicides to provide sustained control of the disease. The study's overall objective was to test the efficacy of various systemic fungicides against CLR in field conditions. During the coffee growing season for two years, various fungicides were tested on two farms in the Kona district of Hawaii. Treated trees showed an 84% reduction in CLR incidence compared to untreated trees in Year 1 and a 79% reduction in Year 2 by the end of the study. Treated trees also had 12% and 38% more total leaves per branch than untreated trees in Years 1 and 2, respectively. All fungicide treatments provided excellent or good control of CLR compared to the untreated control under the climate of Hawaii.