A method of reducing the E/Z isomerization of dimethomorph and a method of reducing the instability of the fungicidal activity of dimethomorph in the presence of light, the methods comprising contacting dimethomorph with fosetyl-aluminium. The fosetyl-aluminium is thought to act as a conversion agent reducing the conversion of the Z isomer of dimethomorph to the E isomer upon exposure to light. In one embodiment, the method employs a composition comprising dimethomorph and fosetyl-aluminium. In another embodiment, the dimethomorph and fosetyl-aluminium are applied separately to a locus to be treated.
