CONTROL OF APPLE POWDERY MILDEW (PODOSPHAERA-LEUCOTRICHA) IN BRITISH-COLUMBIA BY DEMENTHYLATION-INHIBITING FUNGICIDES

In greenhouse studies inoculated McIntosh apple seedlings sprayed with the demethylation-inhibiting (DMI) fungicides myclobutanil, flusilazo le, triadimefon and propiconazole developed significantly fewer powder y mildew colonies than control plants. Fewer colonies were observed on myclobutanil- treated plants than on plants treated with the other fu ngicides. An average of only 0.60 mildew colonies developed on the lea ves of myclobutanil-treated plants within 10 days after inoculation co mpared with an average of 48.2 colonies on the leaves of the control p lants. The DMI fungicides were more effective than thiophanate-methyl in field trials conducted in 1989 and 1990 on infected Jonagold apple foliage. The DMI fungicides flusilazole and myclobutanil effectively c ontrolled foliar powdery mildew under heavy inoculum pressure on Jonat han apple trees in 1987 and 1988, respectively. The cultivars used her ein showed no detectable phytotoxic effects of applying DMI fungicides , neither injury to fruit or foliage nor altered shape or weight.