Identification of a uninucleate Rhizoctonia sp. by pathogenicity, hyphal anastomosis and RAPD analysis

Finnish and Norwegian uninucleate Rhizoctonia sp, isolates. originating from roots of nursery grown conifer seedlings suffering from root dieback, and having Ceratobasidium perfect state, were tested for pathogenicity and genetic related ness. All tested isolates of this pathogen considerably reduced the root system development of Scots pine and Norway spruce seedlings resulting in death or stunted growth. The uninucleate isolates anastomosed readily with each other producing a killing reaction. In a RAPD‐PCR analysis, the uninucleate isolates had different banding patterns from our reference isolates, two Finnish binucleate isolates (AG‐I and R. sp.) and standard tester isolates of genus Ceratobasidium representing anastomosis groups AG‐A, AG‐C, AG‐E, AG‐G and AG‐I. UPGMA analysis clustered the uninucleate isolates together at a greater similarity than 75% while the binucleate isolates formed distinct clusters and were 10‐25% similar to the uninucleate Rhizoctonia sp. Hyphal anastomosis and DNA data suggest that the uninucleate Rhizocionia sp. is an homogeneous group and distinct from the tested binucleate Rhizoctonia isolates.