Rhizoctonia solani damping-off and root rot in oilseed rape and canola

Abstract

An important disease of oilseed rape and canola caused by Rhizoctonia solani is distributed world-wide and poses an economic threat to canola crops in the Prairie regions of western Canada. The population of R. solani that infects oilseed rape and canola is mainly composed of anastomosis groups AG2-1 and AG4 which cause pre- and postemergence seedling damping-off, seedling root rot and basal stem or foot rot (brown girdling root rot) of adult plants. Generally, the isolates of AG2-1 are more virulent than isolates of AG4. Seedling infection by AG2-1 is favoured by cool weather whereas warm weather is conducive to severe damping-off by AG4. The pathogen infects seedling hypocotyls and roots by dome-shaped infection cushions, and macerates the cortical and also the vascular tissues by cell-wall-degrading enzymes. Currently grown cultivars of oilseed rape and canola are susceptible to both AG2-1 and AG4. Sinapis alba, Brassica juncea, B. nigra and older plants of B. napus and B. campestris are less severely infected owing to thicker cuticles and epidermal cell walls. Owing to the lack of adequate genetic resistance in oilseed rape and canola, and the absence of practical methods for suppressing R. solani populations in the field, seed treatment with chemical fungicides (e.g. carboxin, flutolanil, iprodione, tolclofos-methyl, cyproconazole) is the only control available at present for damping-off and root rot. Indigenous populations of Pseudomonas fluorescens, Trichoderma harzianum and the non-pathogenic binucleate Rhizoctonia have demonstrated a certain level of control against the virulent isolates of R. solani in oilseed rape and canola.