Treatment with mycelial extracts, prepared from liquid cultures of Bipolaris oryzae , Pythium ultimum and Rhizopus stolonifer , protected barley (Hordeum vulgare ) against powdery mildew disease caused by the fungus Blumeria graminis f.sp. hordei . The mechanisms of this protection were studied using histopathological methods and molecular analysis. Germination and appressorial formation of B. graminis were generally reduced after treatment with mycelial extracts. Although this reduction (between 12 and 62% depending on treatment and experiment) was inconsistent and only occasionally significantly different from the water-treated control, it indicated a direct antifungal effect of the extracts. In situations where the fungus succeeded in forming an appressorium, penetration efficiency and haustorium formation from these appressoria was not affected - no enhanced penetration resistance associated with papilla formation was detected. However, a post-penetration effect was observed, as B. graminis colonies on mycelial extract-treated leaves produced 50% fewer hyphae than on controls. Northern blot analyses showed earlier accumulation of defence-related gene transcripts following treatment with B. oryzae and P. ultimum mycelial extracts, and to a lesser extent R. stolonifer mycelial extract, compared with water-treated leaves. It is suggested that the protection mechanism of the mycelial extracts involves direct antifungal effects and possible induced resistance for the B. oryzae and P. ultimum mycelial extracts.