Root pathogens play an important economical role in the production of vegetable and ornamental plants. They cause high losses especially in green house production and are difficult to combat by conventional methods, because the use of pesticides in soils or substrates is limited. The application of arbuscular mycorrhiza could be an alternative as mycorrhizal plants generally showed higher resistance against various soil-borne pathogenic fungi. The success of the inoculation is, however, not predictable and mechanisms behind the protective effects are largely unknown. The ornamental crop Petunia hybrida is among other solanaceous plants confronted with attack by root pathogens and was therefore used as model to study mycorrhiza-induced resistance. Different soil-borne fungi were tested and finally Thielaviopsis basicola being pathogenic for more than 130 plant species was used for the pathosystem. Among three different AM fungi only Glomus mosseae BEG 12 turned out to reduce disease symptoms and pathogen spread in the roots. Split root experiments moreover showed that the protective effect was systemic. Expression patterns of genes, which are involved in various pathways of known plant defence responses, were used to test different hypotheses concerning the mechanisms for mycorrhiza-induced resistance. Results indicate the importance of the oxylipin pathway, but priming as basis for systemic resistance induced by plant growth promoting rhizobacteria seems not to play an important role.