First Report and Pathogenicity Assessment of Diaporthe sojae Causing Root Rot on Soybean in Canada

Diaporthe sojae is well known as a causal pathogen of pod and blight as well as seed decay in soybean (Glycine max), it is also known to cause root rot symptoms, though its role is poorly defined. During a 2024 survey in Manitoba, Canada, D. sojae was routinely isolated from roots exhibiting cortical decay and discoloration. The identity of 23 representative isolates was confirmed through morphological characterization and multi-locus phylogenetic analysis using internal transcribed spacer (ITS) and translation elongation factor 1-alpha (EF1-α) sequences. Phylogenetic analysis revealed that the ITS region provided superior resolution in distinguishing D. sojae from the closely related D. longicolla, whereas EF1-α lacked discriminatory power. Pathogenicity was evaluated for 19 isolates on soybean cv. Sperling in a greenhouse assay using colonized wheat-kernel inoculum. While all isolates were pathogenic, significant virulence diversity was observed among isolates, with Disease Severity Indices (DSI) ranging from 68.8% to 100%. Two distinct virulence phenotypes were identified: acute virulence isolates causing 100% seedling mortality (damping-off), and more chronic phenotype characterized by root necrosis and stunting without immediate plant death. This study constitutes the first confirmed report of D. sojae causing soybean root rot in Canada. These findings expand the known disease spectrum of D. sojae and highlight the necessity of including this pathogen in future root-rot diagnostic and management strategies.