Characterization and Identification of Asexual Strains of Pythium Associated with Root Rot of Rose in Japan

This study was conducted to survey the distribution of asexual isolates of Pythium in rose production and to characterize and identify them. Asexual isolates with proliferating globose sporangia belong to group P according to the key of van der Plaats‐Niterink (1981; Monograph of the genus Pythium. Studies in Mycology, Vol. 21, Centraalbueau Voor Schimmelcultures, Baarn, The Netherlands). Group P isolates were recovered from rotted roots of both cutting and miniature roses cultured in rock wool and ebb‐and‐flow culture systems, respectively, throughout the main rose production area of Japan. The typical feature of the P group isolates was that they could grow fast at high temperature, at least 30 mm per 24 h at 35°C. There was no difference between the P group isolates and P. helicoides in morphology and size of sporangia and sporangial germination mode. The symptoms caused by the group P isolates were root rot, followed by leaf blight and plant death in severe cases. In restriction fragment length polymorphism analysis of the rDNA‐ITS region, the banding patterns with five of six enzymes were identical between group P and P. helicoides, the only difference being seen with HhaI. In direct amplification analysis of minisatellite‐region DNA with M13 primer, group P and P. helicoides shared three of five distinct bands. In contrast, P. oedochilum and P. ostracodes showed different banding patterns except for each one band. The results suggest that the group P isolates obtained from rose root rot may be asexual strains of P. helicoides.