Abstract
SUMMARY: Evidence for the occurrence of the ‘Trichoderma effect’ in nature is reviewed. It is considered that the effect may be a specifically evolved defence mechanism, and that root inhabiting species of Phytophthora may be more ecologically specialized for response to Trichoderma than others. It is suggested that the ‘Trichoderma effect’ and similar phenomena could influence speciation in heterothallic Phytophthora species.