Saprolegniasis is considered one of the most important threatening disease problem facing aquaculture specially Nile tilapia (Oreochromis niloticus). This study aimed to conduct mycological study on saprolegniasis in El-Manzala fish farm in Dakahlia province. The incidence rate of saprolegniasis in El-Manzala fish farm was 90.9%. All strains isolated were morphologically classified in the genus Saprolegnia. They were identified as follow S. diclina oogonia are spherical or pyriform, with spherical central oospore and diclinous antheridial branches. S.ferax oogonia spherical with centric or subcentric oospore with no antherdial attachment. S. australis oogonia, spherical or pyriform, unpitted with spherical subcentral oospore and diclinous antheridial branches. PCR yielded a single specific and clear amplified band of suspected size 700bp corresponding to ITS genes.Histopathological examination of skin of naturally infected fish O. niloticus showed necrosis and desquamation in the superficial layer of epidermis with edema and heterophilic infiltration in the underlying dermis, in addition reddish to brownish fungal particles appeared on skin sections. Gills revealed congestion of lamellar blood capillaries, goblet cells metaplasia and desquamation of secondary lamellae.
Keywords: Nile tilapia, Saprolegnia, Incidence, Histopathological