Abstract
Twenty-four isolates of Saprolegnia were obtained from fish farms, and the relationship between pathogenicity to rainbow trout (Oncorhynchus mykiss) and their biological characteristics was studied. Fifteen isolates identified as Saprolegnia parasitica were classified into three groups (Groups 1, 2 and 3) according to the mortalities following artificial infection. The isolates belonging to Group 1 which showed high pathogenicity to rainbow trout produced many characteristically catenulated gemmae, but the isolates of the other groups did not. Sensitivity to cycloheximide of the isolates of Groups 1 and 2 was higher than that of Group 3. Nine isolates identified as S. diclina were demonstrated to be saprophytic. Sensitivity of S. diclina to polyphenon- 100R was lower than that of S. parasitica. These results suggest that Saprolegnia parasitica and S. diclina can be practically distinguished by the pathogenicity to fish and some biological tests.
