Characterization, Virulence and Genetic Variation of Rhizoctonia solani AG-1 IA in India

Variability of 35 isolates of Rhizoctonia solani Kühn [Teleomorph: Thanatephorus cucumeris (Frank) Donk] causing sheath blight of rice and weeds, was examined by polymerase chain reaction-random amplified polymorphic DNA (PCR-RAPD) analysis in conjunction with morphology and pathogenicity studies. Studies on morphological characterization of R. solani isolates showed that isolates were highly variable both in mycelial and sclerotial parameters, with no consistent characters related to virulence, origin, production system or geographic origin. Pathogenicity of R. solani was also evaluated and based on their virulence, isolates were classified in to different virulent groups. The similarity values of RAPD profiles ranged from 0.37 to 0.85 with an average of 0.61 among the isolates. The percentage polymorphism detected per primer varied from 76.9 to 100%. Amplification of RAPD primers, viz., 723, 732, 739, 743, 744 and 750 resulted in 100% polymorphism, which suggested their utility in the individual DNA finger printing of the sheath blight isolates. The cluster analysis using unweighted paired group method with arithmetic averages could distinguish MTU-w isolate from rest of the isolates, which was in good agreement with morphological (non-sclerotia producing) and pathogenicity (absence of typical sheath blight symptoms) characters. Further, relation between cultural/morphological characteristics, aggressiveness and genetic variation based on RAPD markers, and possible reasons for high intra-group variability are discussed.