In a study on leaf-inhabiting tetranychid mites (Tetranychus urticae Koch–TSSM and Panonychus ulmi (Koch)–ERM) we investigated the effects of an extrinsic factor on the mites environment, namely phylloplane fungi. In a research orchard four trees were selected and treated with an aerosol application of a phylloplane fungus (Alternaria alternata) in a tap-water emulsion. Applications were made immediately after each sampling, with the exception of the last sample date. Two tap water controls for each treated tree were also sampled: a nearest neighbour (< 3 m from the treated trees) and a distant neighbour (> 30 m from the treated trees with other apple trees in between). Due to possible migration from the treated trees to near neighbours, the distant control best reflected normal orchard conditions. Eight samples were taken throughout the 1994 growing season; however, appreciable mite populations were only observed on the last four sample dates. On the treated trees, the ERM maintained a steady low population (less than ten per leaf) whereas the TSSM showed a population outbreak (up to 44 mites per leaf). Conversely, on the distant trees, the TSSM maintained a low population (less than ten per leaf) while the ERM showed an outbreak (up to 33 per leaf). Observing on a leaf by leaf basis, when tetranychids were present on a leaf, either one species dominated or the other, suggesting mutual competitive exclusion, the outcome of which was reversed to favour TSSM on trees that received an application of fungus. We concluded that the application of additional or supplemental amounts of A. alternata to apple leaves enhanced the population growth of TSSM compared to that of ERM. Possible mechanisms are discussed.