Uni‐ and binucleate Rhizoctonia spp. co‐existing on the roots of Norway‐spruce seedlings suffering from root dieback

Summary Rhizoctonia fungi were isolated from the roots of 2‐year‐old nursery‐grown Norway‐spruce seedlings displaying root‐dieback symptoms. The most frequently isolated species, a uninucleate Rhizoctonia sp., was found to co‐exist with binucleate Rhizoctonia in the same root system of several seedlings. All the uninucleate isolates anastomosed with each other forming a single anastomosis group with common cultural characteristics. Binucleate Rhizoctonia isolates were divided into several, morphologically dissimilar anastomosis groups (AG‐I, R. spp.). In a pathogenicity test under sterile conditions, isolates belonging to the uninucleate Rhizoctonia sp. infected all root regions, particularly the root tips, resulting in a stunted root‐system morphology, as was also observed in the isolation material. Binucleate Rhizoctonia spp. colonized only basal root regions, occasionally infecting cortical cells with monilioid hyphae, and had no effect on root growth.