Prevalence, pathogenicity and cultivar resistance of Fusarium and Rhizoctonia species causing soybean root rot

Zhang, J. X., Xue, A. G., Cober, E. R., Morrison, M. J., Zhang, H. J., Zhang, S. Z. and Gregorich, E. 2013. Prevalence, pathogenicity and cultivar resistance of Fusarium and Rhizoctonia species causing soybean root rot. Can. J. Plant Sci. 93: 221-236. Root rot complex, caused by Fusarium and Rhizoctonia species, is a major soybean disease in Canada. We isolated nine Fusarium and Rhizoctonia species including F. oxysporum (Fo), F. graminearum (Fg), F. solani (Fs), F. avenaceum (Fa), F. tricinctum (Ft), F. sporotrichioides (Fsp), F. equiseti (Fe), F. poae (Fp), and R. solani (Rs) from soybean roots in eastern Ontario, Canada. The isolation results indicated that Fo was the most prevalent species while Fa, Fsp, and Fp were the least frequent species in the soybean rhizosphere. Numbers of Fo, Fs, Fg, and Rs isolates recovered from adult plant roots were significantly greater than those from seedling roots (P < 0.01). The Rs, Fg and Fsp isolates were significantly more abundant in the no-till field than in the tilled field (P < 0.01). Based on the greenhouse assays, Rs, Fg, and Fa were the most pathogenic species, while Fe and Fsp were the least pathogenic to soybean. The field resistance evaluation, based on the root rot severity, identified 21, 17, 30, and 3 out of 70 cultivars among the most tolerant to Fg, Fo, Fs, and Rs, respectively. A few of the cultivars showed partial resistance to multiple species, based on root rot severity and reduction in their seedling emergence, plant height, and root dry weight, but no cultivar was found to partially resist all four species. There was no correlation (P > 0.05) between root rot severity and the reduction in seedling emergence, plant height, or root dry weight.