The morphology and physiology (temperature-growth relationships) of seventeen isolates of Saprolegnia from fish hatcheries in Japan were compared. A linear relationship between the number of hairs per bundle against bundle length, on secondary zoospore cysts, was found. One isolate of S. parasitica, a particularly strong pathogen, from silver salmon, had a secondary zoospore cyst ornamentation consisting of separate bundles of long, short and intermediate length hairs. This feature was also seen to a lesser extent in other isolates, including a strong pathogen from brown trout in the U.K.
Using the Japanese isolates, temperature-growth relationships at 30 °C distinguished isolates of S. parasitica, from rainbow trout, from those from other fish. A mutant of one of the rainbow trout isolates showed differences in growth rates compared with the parent isolate, the possible ecological significance of which is discussed.
Mycological Research (1990) 94 (2) 182-190 [doi:10.1016/S0953-7562(09)80611-4]
Some characteristics of Saprolegnia obtained from fish hatcheries in Japan