Process for combating fungi

A process for combating overwintering powdery mildew in trees comprising spraying said trees while in a dormant phase, with an antifungally effective amount of an aqueous composition consisting essentially of water and at least 0.5% by volume of an active ingredient which is an anionic surface active agent selected from the group consisting of sodium lauryl sulphate, sodium dodecylbenzenesulphonate, butylnaphthalene sulphonate, a mixture of the sodium salts of diisopropyl- and triisopropyl-naphthalene sulphonic acids, and sodium dioctyl sulphosuccinate.