Intraspecific DNA polymorphisms of Pythium irregulare

Forty-seven isolates of Pythium irregulare from different hosts and geograp hic origins were compared from molecular, morphological and physiological v iewpoints. They were divided into four groups (I-IV) based on ITS-RFLP anal ysis and RAPD analysis. Groups I and II included 32 and eight isolates, res pectively, collected from diverse hosts and geographic origins, and groups III and IV comprised seven isolates derived from sugar beet and sugar beet field soil. Group I had smaller oogonia and oospores than did the other thr ee groups. In groups I and II, a significantly higher percentage of the oog onia produced multiple projections compared to groups III and IV which occa sionally produced one projection. The growth rate of the four groups was si milar at 5-30 degreesC. At 33 degrees, many isolates of group I grew rapidl y but most of the isolates of other groups grew slowly, and at 35 degrees, the former grew but the latter did not. In phylogenetic analysis based on s equences of the ITS region, four groups of P. irregulare were included in o ne cluster with P. sylvaticum. Groups I-II and III-IV clustered more tightl y in the same branch, respectively. The genetic divergence between I-II and III-IV was higher than between each group (I-II and III-IV) and P. sylvati cum, indicating that groups I-II and III-IV may represent two different spe cies.