In the present study, characterisation of genotypic variations of Rhizoctonia solani AG-1 IA associated with rice sheath blight by Rep-PCR assay and their structure of the genotypic variations by monitoring vertical and horizontal movements of their populations at a short distance level were investigated in Japanese paddy fields. Differences of the Rep-PCR fingerprintings were observed and distinguished into four genotypic variations referred to as GI, GII, GIII and GIV, respectively. Although similarity index of each genotype showed high levels of homology (85-90%) within the same genotypes, low levels of similarity index (65-70%) were also varied among the comparison of different genotypes. Moreover, diversity of genotypic populations was observed which is consistent with the correlations between the geographical undulations of the paddy fields and the occupation of their genotypic populations, indicating the presence of genotype GI on low lands such as AK1 and also the presence ofgenotype GIV on high lands such as AK4.
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