Abstract: Oilseed rape (OSR) grown frequently in rotation suffers from yield decline. Moleculartechniques have been used to identify changes in the rhizosphere fungal communities of OSRgrown in different rotational frequencies with wheat. In particular, Olpidium brassicae andPyrenochaeta sp. were found to increase in relative abundance in rhizosphere fungalcommunities of OSR cropped continuously or in short rotation, compared to OSR grown in longrotations. In order to investigate whether these two microorganisms contribute to yield decline,working cultures were isolated from field samples and controlled glasshouse experiments werecarried out. For this glasshouse work, a model plant species (Brassica oleracea; closely related toOSR) was used to facilitate rapid seed production under glasshouse conditions. Experiments wereinitially carried out in a soil-less medium (sand-terragreen mix) to assess the impact of theindividual fungal species alone on the plants. Plants were inoculated with a range of doses of thetwo fungi in separate experiments, and results from this model system indicated that both speciesaffected plant growth in terms of either reduced branching (O. brassicae) or reduced seed qualityand quantity per pod (Pyrenochaeta sp.).