AG-5 II: A Novel Rhizoctonia solani Anastomosis Subgroup of AG-5 on Legumes and Sugar Beet in Sweden

This study focuses on eighty-one Rhizoctonia solani Kühn. anastomosis group (AG)-5 isolates collected from various field crops, primarily Pisum sativum L. (pea), in southern Sweden and other (Nordic) countries since 2015. Using rDNA internal transcribed spacer (rDNA-ITS) sequencing and classical hyphal anastomosis assays, two distinct subgroups of AG-5, named AG-5 I and AG-5 II, were identified. ITS analysis revealed that the thirty-four AG-5 II isolates (42% of all AG-5 isolates) exhibited higher intra-isolate polymorphism, with numerous insertions and double peaks, suggesting ongoing evolutionary changes and a greater diversity of ITS regions within nuclei. In contrast, tef1-α sequences of a representative subset of AG-5 isolates showed less polymorphisms, higher bootstrap support, and clearer clustering of the two subgroups, highlighting the stability of this marker for phylogenetic inference. Pathogenicity assays showed that AG-5 II caused more severe epicotyl discoloration in pea and faba bean, while AG-5 I was associated with higher incidence of damping-off in sugar beet. AG-5 II also displayed higher tolerance to fludioxonil, but the broader ecological and evolutionary significance of this observation requires further investigation. In conclusion, this work provides the first report on the subdivision of AG-5 into two subgroups, AG-5 I and the evolving AG-5 II. rDNA-ITS and tef1-α markers together allow robust delineation of these subgroups, which differ in pathogenicity and fungicide tolerance, emphasizing their relevance for crop disease management.