Cultural and pathogenic variation in potato isolates of Rhizoctonia solani in Northern Ireland

One hundred and eighty two isolates of R. solani from sclerotia on tubers of various potato cultivars grown in 24 different locations in N. Ireland showed wide variation in cultural characteristics. The pattern of sclerotial formation, being the most stable and well-defined character of each isolate, was used to divide them into six groups. Attempts to anastomose these with test strains of anastomosis groups (AG-1, AG-2 type 1, AG-2 type 2, AG-3, AG-4 and AG-5) showed that all isolates in N. Ireland groups 1–5 anastomosed with AG-3 and those in group 6 only with AG-2 type 1. Mycelial growth of group 6 was slower and hyphal diameter smaller compared with that of other groups. Pathogenicity tests on 5 potato cultivars (Arran Banner, Arran Consul, Bintje, Dunbar Standard and Pentland Dell) showed that the isolates in groups 1–5 (AG-3) caused significantly more stem canker and black scurf than those of group 6 (AG-2 type 1).