Multitrophic interactions between a Rhizoctonia sp and mycorrhizal fungi affect Scots pine seedling performance in nursery soil

Interspecific variation in ectomycorrhizal fungal (ECMF) control of a root pathogenic uninucleate Rhizoctonia sp. (UnR) was identified in vitro and in planta. Fungal-fungal and host-fungal interactions were assessed in direct confront ation and cell-free assays, the rhizosphere of Scots pine (Pinus sylvestris ) seedling radicles and seedling mycorrhizospheres developed in N-limited n ursery soil. Isolates of Suillus bovinus inhibited UnR growth although no agar-diffusabl e fungicidal activity was detected. Presence of nonsymbiotic ECMF mycelia d id not prevent UnR colonization of radicle apices and the onset of damping- off symptoms. Seedlings hosting S. bovinus mycorrhizas and extensive extram atrical mycelium showed vigorous and healthy shoot growth after a 168-d UnR challenge. Root biomass of Wilcoxina mikolae and Paxillus involutus coloni zed seedlings were negatively affected by both low soil nutrient status and UnR exposure. However, UnR was isolated from long and mycorrhizal short ro ots in all ECMF coinoculation treatments. The differential responses highlight multitrophic host-fungal interaction d ynamics that require further characterization in the development of 'effici ent' UnR biological control solutions utilizing mycorrhizal fungal inoculan ts. (C) New Phytologist (2001).