Occurrence of Pythium Sp. on the Marine Alga Porphyra

SUMMARY

Pythium sp., as a parasite of the algal genus Porphyra, is reported from the Puget Sound in Washington; Monterey, California; and Sandwich, Massachusetts. Previously this host-parasite relationship was known only from Japan. In all infections observed, the macroscopically visible lesions are circular and lighter than the healthy algal tissue. The coenocytic hyphae grow by or through the cells of the host. Sporangium production and the conditions which promote the process are described. In all cases the sporangia are nematosporangia. Since oogonia have not been seen, the fungus is referred to Pythium sp. Hyphae of the West Coast fungi which are readily grown in pure culture have been used to artificially infect Porphyra sp. in the laboratory. Observations on the ecological role of this fungus are made but no conclusions as to its importance as a parasite are possible at this time.