Virulence spectrum and genetic structure of Rhizoctonia isolates associated with rice sheath blight in the northern region of India

Rhizoctonia solani Kuhn, a highly variable pathogen, causes sheath blight of rice globally leading to serious yield losses under favourable environmental conditions. Sixty-four Rhizoctonia isolates collected from diverse rice growing agro-ecological zones of the north Indian state of Punjab were analyzed for species identification, genetic diversity, morphological features and virulence pattern. Accurate identification of the fungal species using internal transcribed spacer species-specific primers confirmed presence of R. solani (84.4 %), R. oryzae-sativae (10.9 %), R. oryzae (1.5 %) and mixed infection of these Rhizoctonia sp. Genetic structure of the isolates resolved using inter simple sequence repeat primers revealed high degree of polymorphism (PIC value ranging from 0.80 to 0.90), clustering the pathogen population into four major groups. The virulence pattern of 18 isolates representing genetically diverse groups generated by ISSR profile was quantified on seven rice genotypes/cultivars i.e., Tetep, Jasmine 85, Te-Qing, D-256, D-6766, PR108 and PAU201 with different levels of resistance to rice sheath blight. The hierarchical cluster analysis based on different disease variables classified isolates into six main groups. Reaction of individual isolate to each genotype revealed different levels of virulence pointing towards high evolutionary potential, thus enabling it to adapt to diverse geographical regions.