Field efficacy of fungicides to control downy mildew of onion

Downy mildew is the main leaf disease of onion in southern Brazil. The management of the disease is carried out through frequent applications of fungicides. The objective of this study was to evaluate the fungicide efficiency in controlling downy mildew, under field conditions, during 2016–18 crop seasons. We evaluated the effect of weekly sprays of different active ingredients on the area under disease progress curve (AUDPC) and on commercial yield. There was variation among the best treatments in the three years of the test. However, we observed that sprays with metalaxyl-M (100 g/ha) + mancozeb (1600 g/ha) or mancozeb (1875 g/ha) were always allocated to the most productive groups: i) metalaxyl-M + mancozeb in the most productive group in 2016 (41.61 ton/ha), in the most productive group in 2017 (21.25 ton/ha), and in the second most productive group in 2018 (16.70 ton/ha); ii) mancozeb in the second most productive group in 2016 (32.62 ton/ha), in the most productive group in 2017 (19.88 ton/ha), and in the second most productive group in 2018 (16.00 ton/ha). The results demonstrated that metalaxyl-M and mancozeb are important molecules in the management of the disease. The use of fungicides composed only of molecules with systemic action is not recommended for the management of onion downy mildew under Brazilian conditions.