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Distribution of Pythium porphyrae, the Causal Agent of Red Rot Disease of Porphyrae spp., in the Ariake Sea, Japan

Distribution of Pythium porphyrae, the causal agent of red rot disease of Porphyra spp., in seafloor sediment was investigated in the Ariake Sea, Japan. A total of 170 samples of each 200 ml of sediment was collected from the seafloor at a total of 13 sites across the sea from 1998 to 2002. Each sample was filtered through two layers of nylon mesh with pore sizes of 100 and 15 μm. The residue on 15 μm mesh was assayed by a soil plating technique using a semiselective medium for P. porphyrae and by polymerase chain reaction (PCR) using species-specific primers. P. porphyrae were detected in 6 out of 13 sites and 2 out of 10 sites surveyed by soil plating and PCR, respectively. The representative isolate of P. porphyrae from the sediment was identical to the Porphyra thallus isolate from the same sea based on pathogenicity to the thallus, morphology, and rDNA internal transcribed spacer sequences. Recovery of P. porphyrae propagules in the sediment was up to 60 CFU per 100 ml of the fresh sample and was consistently higher in May than in the other months. The results suggest that P. porphyrae is distributed in the seafloor sediment in a wide area of the Ariake Sea.