| Pythium is a large genus of the class Oomycetes including more than 80 species, some of which are important plant pathogens with worldwide distribution. As overlapping similarities in the morphological characteristics of Pythium species, as well as intraspecific variation in these characteristics, it can make accurate classification for Pythium isolates difficult. As a consequence, combining molecular phylogeny with morphological characters, it is the better view of the taxonomy and phylogeny of the Pythium.1. Eleven species were identified from 34 Pythium strains which were isolated from Hangzhou suburb. They were P. spinosum Sawada, P. ultimum var. ultimum Trow, P. debaryanum Hesse, P. sylvaticum Campbell & Hendrix, P. irregulare Buisman, P. oligandrum Drechsler, P. myriotylum Drechsler , P. dissotocum Drechsler , P. graminicola Subramanian, P. acanthicum Drechsler and P. aphanidermatum (Edson) Fitzp.. The P. sylvaticum was a new record of Pythium to China. The P. myriotylum was a new record of Pythium to Zhejiang province. P. spinosum and P. ultimum were predominant species as pathogens in the suburb of Hangzhou, and the isolating frequencies were at 29.4% and 17.6%, respectively.2. The sequences of nrDNA ITS of 67 isolates representing 58 species Pythium and 6 species representing 6 groups of Phytophthorta were analyzed, and their phylogenetic tree was reconstructed with Halophytophthora batemanensis as an outgroup, using neighbor-joining method. The results showed as the following:(1) The present results showed that Pythium ostracodes, P. chamaehyphon grouped in a clade together with P. montanum, P. carbonicum and P. vexans, which were between other Pythium species and 6 species of Phytophthorta. The five species of Pythium all produced spherical zoosporangia. They still were grouped in a clade together with other Pythium species, the bootstrap values is at 67%. The author suggest it is better to consider them as "border species", and not support them as " separated genus".(2) Pythium undulatum transferred into the genus Phytophthorta (as Phytophthorta undulatum) formed a single group. In the phylogenetic tree, P. undulatum was grouped into one clade together with other 52 species of Pythium, as the bootstrap values is at 100%. It showed that the relationship of P. undulatum was closer to the genus Pythium than Phytophthorta. So it was much better to consider P. undulatum as a species of Pythium, rather than Phytophthora.(3) The 52 of 58 species Pythium are further divided into two clades whose branching patterns were strongly supported by high bootstrap values. Clade I was comprised of 31 species, 30 of which produced spherical zoosporangia. Clade II was comprised of 21 species, 19 of which produced filamentous to lobulate zoosporangia. The results showed that the relationship between Pythium phylogeny and its zoosporangia morphology was very close. The zoosporangia is the most important morphological characteristics with taxonomic sense, and others such as oogonia ornamentation, antheridia attachment, number of antheridia, heterothallism were polyphyletic inferred from ITS sequence analysis.(4) The similarities of the ITS sequences of Pythium species isolated from Hangzhou suburb with their type isolates or representative isolates were compared. Some of which are higher, bootstrap values are at 100% , such as P. sylvaticum BXS9, P. spinosum SNP10, P. graminicola PHP66, P. myriotylum PA1, P. dissotocum BXS42 and P. aphanidermatum DC 16. Some of which are lower, bootstrap values are only 78%, such as P. oligandrum RCU1 and P. oligandrum NZWM07 from New Zealand.3. The relationship between P. ultimum Trow var. sporangiiferum and P. ultimum Trow var. ultimum based on ITS sequence was studied. The results showed: the average of G+C contents in ITS of P. ultimum Trow var. sporangiiferum was lower than that of P. ultimum Trow var. ultimum, the former was at 41.3%, the latter was at 42.5%; the mean length of the ITS fragment of the former was 829bp, and that of the latt...
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