Identification Pathogenicity Distribution and Chemical Control of Rhizoctonia solani Causing Soybean Root Rot in Northeast China

Soybean root rot caused by Rhizoctonia solani is a yield-limiting disease in Northeast China, particularly under continuous monoculture and cool climatic conditions. Despite its agronomic impact, the epidemiology and fungicide resistance profile of the pathogen remain inadequately characterized. In this study, a comprehensive survey conducted in Heilongjiang Province yielded 990 pathogenic isolates belonging to 11 fungal species. Among them, 55 strains were identified as R. solani based on combined morphological and molecular analyses. These isolates induced typical symptoms of root and stem browning with constriction. Pathogenicity tests on 30 R. solani isolates indicated that 83.3% were highly pathogenic. The pathogen exhibited a distinct geographic distribution, with the highest percentage of pathogen isolation recorded in Jiamusi (26.6%), which accounted for 61.8% of all R. solani isolates. In vitro fungicide sensitivity assays demonstrated that fludioxonil and prochloraz were highly effective (EC50 < 0.0050 µg·mL−1), whereas resistance was observed to tebuconazole, difenoconazole, pyraclostrobin, and carbendazim. Pot experiments confirmed that fludioxonil seed treatment (15 g a.i./100 kg seeds) provided superior control efficacy (63.07%) compared to prochloraz (46.85%). These findings establish R. solani as a dominant causal agent of soybean root rot in the region and support the prioritized use of fludioxonil for sustainable disease management. By elucidating the pathogenicity, distribution, and resistance patterns of R. solani, this study provides critical insights for controlling soybean root rot in cold-climate production systems and facilitates the development of targeted management strategies.