A total of 40 isolates composed of 24 isolates of R. solani, 10 isolates of R. oryzae, 3 isolates of R. oryzae-sativae and 3 isolates of S. hydrophilum were used to study the phylogenetic relationships of rice sheath diseases. According to the phylogram, four distinct groups were clustered together as S1, S2, S3 and S4. DNA sequence analysis showed that all of the isolates tested formed the distinct groups corresponding to the particular species concerned. All of the isolates in S 1 group were composed of R. solani sequences and are clustered with the bootstrap value of 92. S2 group was composed of 10 sequences of R. oryzae isolates and three sequences of R. oryzae-sativae isolates with the bootstrap value of lower than 50. S3 group was composed of three sequences of S. hydrophilum. S4 group containined the outgroup sequence, AM503536. We found that the highest evolutionary divergence at R. solani compared with the other three species, R. oryzae, R. oryzae-sativae and S. hydrophilum. The highest evolutionary divergence, 11.9, was observed between the sequences of R. solani isolates and R. oryzae-sativae isolates. The lowest divergence, 2.66, was observed between R. oryzae and S. hydrophilum isolates. The evolutionary divergence within species was highest in R. oryzae sequences, which has a value of 0.80. However in S. hydrophilum isolates, the evolutionary divergence of 0.00 was estimated.
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