Identification and asexual reproduction characterization of a Saprolegnia ferax pathogen from the egg of Yellow catfish(Pelteobagrus fulvidraco)

[Objective] The aim of the present study was to isolate and identify the pathogen of Pelteobagrus fulvidraco eggs suffering from saprolegniasis,and study its asexual reproduction characteristics.[Methods] Filamentous fungal strains were first isolated from the Pelteoba-grus fulvidraco eggs suffering from saprolegniasis using the traditional method.The patho-genic strain was further confirmed through artificial infection experiment,and identified by using morphological observation and phylogenetic analysis based on its ITS rDNA sequence.Additionally,its asexual reproduction characteristics was studied using single factor method.[Results] Four filamentous fungal strains were isolated form Pelteobagrus fulvidraco eggs with saprolegniasis,and strain HP was proved to be pathogenic to Pelteobagrus fulvidraco eggs by artificial infection.Therefore,morphology and asexual reproduction characteristics of strain HP were studied,and the phylogenetic analysis based on its ITS rDNA sequence was further conducted.The experimental results showed that the hyphae of strain HP were aseptate,transparent and seldom branched.Its zoosporangia were often clavate and renewed internally.Primary zoospore was discharged in Saprolegnia fashion.New sporangium generated from the base of old sporangium by the way of internal proliferation.Spherical oogonia were attached by monoclinous or diclinous antheridium hyphae.The ITS rDNA sequences of strain HP were naturally clustered with ITS rDNA sequences of Saprolegnia sp.in GenBank with 99% of homology,and had closest relationship with Saprolegnia ferax strain Arg4S(GenBank acces-sion number: GQ119935).Combined morphological characterization with phylogenetic analy-sis based on ITS rDNA sequence,strain HP was identified as Saprolegnia ferax.In addition,strain HP could produce zoospores at 5 °C?35 °C and pH 4?10,its optimum temperature and pH for the zoospore production were 20 °C and 7,respectively.It was greatly inhibited by 5?25 mg/L formalin and 0.25?1.25 mg/L dithiocyano-methane.[Conclusion] The pathogen of Pelteobagrus fulvidraco egg saprolegniasis was isolated and identified in the present study,and its asexual reproduction characteristics was also determined,which could serve as a foun-dation for the control of Pelteobagrus fulvidraco egg saprolegniasis.