Suppression of Cedar Apple Rust Pycnia on Apple Leaves Following Postinfection Applications of Fenarimol and Triforine

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Abstract
The experimental fungicides fenarimol and triforine were compared with mancozeb to determine control of cedar apple rust (caused by Gynmosporangium juniperi-virginianae) following postinfection applications to apple [Malus pumila] under both greenhouse and orchard conditions. Fenarimol and triforine inhibited normal development of pycnia and only small flecks developed. In the greenhouse, fenarimol at 20, 40 and 60 .mu.g/ml provided 98% control when applied 3 days after inoculation; triforine at 120, 240 and 360 .mu.g/ml provided 89-98% control 1 day after inoculation with 59-94% and 68-83% control after 2 and 3 days, respectively. In the orchard, fenarimol at 20 and 40 .mu.g/ml suppressed pycnial formation 96-99% and 97-100%, respectively, when applied 5 or 7 days after commencement of infection periods. Triforine at 240 .mu.g/ml suppressed pycnial formation by 80-83% with sprays within 2 days after infection in the orchard, but control decreased to 71 and 79% at 3 or 5 days. Inadequate control was obtained with postinfection applications of mancozeb, a commonly used rust fungicide, in either greenhouse or orchard.