Identification of an isolate of Saprolegnia ferax as the causal agent of saprolegniosis of yellow catfish (Pelteobagrus fulvidraco) eggs.

Saprolegnia species have been implicated for significant fungal infections of both living and dead fish as well as their eggs. In the present study, an oomycete water mould (strain HP) isolated from yellow catfish (Peleobagrus fulvidraco) eggs suffering from saprolegniosis was characterized both morphologically and from ITS sequence data. It was initially identified as a Saprolegnia sp. isolate based on its morphological features. The constructed phylogenetic tree using neighbour joining method indicated that the HP strain was closely related to Saprolegnia ferax strain Arg4S (GenBank accession no. GQ119935), that had previously been isolated from farming water samples in Argentina. In addition, the zoospore numbers of strain HP were markedly influenced by a variety of environmental variables including temperature, pH, formalin and dithiocyano-methane. Its zoospore formation was optimal at 20 °C and pH 7, could be well inhibited by formalin and dithiocyano-methane above 5 mg/L and 0.25 mg/L, respectively. To our knowledge, this is the first report on the S. ferax infection in the hatching yellow catfish eggs.