Abstract
A saprolegniasis occurred in a group of rainbow trout fingerlings of about 1.2 g in body weight at the Koide branch of the Niigata Prefectural Freshwater Fisheries Experimental Station. The initial sigh of the disease was an appearance of fin rot. Saprolegnia sp. was thought to infect secondarily on the area of fin rot.
Six strains of the fungus were isolated from the base of the dorsal fin of 6 diseased fish. Inoculation experiments using moon fish showed that these six strains differed to some extent in pathogenicity from each other (Table 2.). Among the 6 strains strain N7619 was most virulent.
All the strains were classified into Saprolegnia sp. according to SPARROW (1960) and DICK (1973) from the characteristics of the asexual organs on hemp seeds. Of the 6 strains, only strain N7619 formed sex organs. In this strain diclinous antheridial branches were predominantly produced, and the oogonial wall were pitted. The type of internal structure of oospores was subcentric. From these structures the strain was identified as S. australis according to SEYMOUR (1970). The characteristics of the sex organs of the strain also agreed with those of S. australis described originally by ELLIOTT (Table 3.).
