Diversity of <i>Rhizoctonia solani</i> anastomosis groups on field crops in Sweden and their virulence on legumes

Rhizoctonia solani is a soil-borne pathogen with a broad host range and considerable genetic diversity, yet little is known about its population structure in Sweden or Northern Europe. This study presents the most comprehensive survey to date of R. solani and related fungi across major field crops in Sweden, focusing on the southern agricultural region Scania. A total of 202 isolates were collected and characterized using rDNA-ITS sequencing for anastomosis group (AG) identification, geographic mapping, and multivariate analysis. The isolates primarily belonged to multinucleate AG-5 (81), followed by AG-3 (32), AG-2-1 (30), and AG-1 IB and AG-4 HGII (both 13), among other AGs, binucleate isolates, and Waitea species. AG-5 was clearly dominant, particularly on legumes, showing mild aggressiveness on pea and faba bean and high intraspecific ITS diversity. In contrast, AG-4 HGII and AG-2-1 displayed higher virulence, while AG-3 was non-pathogenic on the same hosts. Multivariate analyses revealed crop- and region-specific AG distributions. AG-3 was strongly associated with carrot and potato in Northeastern Scania, whereas across the rest of the region, AG-5 dominated legumes and was also found on sugar beet, together with AG-4 HGII and AG-2-1, while AG-2-1 occurred mainly on Brassicaceae. These findings have direct implications for crop rotation planning and disease management, highlighting potential risks in rotations involving legumes, Brassicaceae, and sugar beet, while rotations with carrot or potato may present lower risk. Overall, this study underscores the ecological complexity of Rhizoctonia in Swedish agroecosystems, emphasizing the need for localized surveillance and tailored management strategies to mitigate disease risk.