Plant protection against soil-borne diseases has been a challenge in horticultural production of Ericaceae for a long time. We tested the hypothesis that ericoid mycorrhizal fungi (ERMF) control root pathogens. Isolates of ERMF previously obtained from Calluna vulgaris and Rhododendron hirsutum were selected based on growth inhibiting activity against Oomycetes in dual agar plate tests. In addition, we assessed their impact on economically important Pythium spec. and Phytophthora cinnamomi in sterile Rhododendron plantlets in a liquid culture system, which is suitable for continuous observations of the infection process. For this purpose, rooted cuttings of micro-propagated Rhododendron plantlets were transferred to a mineral solution and subsequently inoculated with either Oidiodendron maius or the ERMF isolates. Before the root pathogens were applied to the experimental system, the symbioses were established over a four-week period. Mycorrhizal development, pathogen infection sites and development in mycorrhizal plants and non-mycorrhizal control plants were assessed microscopically. The root-colonising intensity of the tested ERMF differed considerably, but each of the applied ERMF impaired external pathogen mycelium and reduced pathogenic infections. A complete reduction was achieved at higher ERMF colonisation levels. The failure of symbiosis formation allowed pathogens to infect and spread. The quantification data concerning mycorrhiza frequencies and extramatrical hyphal nets provided details for a discussion on the suppressive effects of ERMF on the pathogens. The tested ERMF confer at least a localized protection from pathogen attack through suppression prior to infection. It is now to prove, whether these biocontrol effects will also be expressed in pot experiments.

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