Fungal strain
One isolate of Saprolegnia used in challenge experiment; Saprolegnia ferax isolate. It was isolated from skin lesions of Nile tilapia suffered saprolegniosis from different fish farms in Egypt. Isolates were identified according to their morphological and sexual character and sequenced (unpublished data). Fungal isolates were cultured on glucose yeast extract (GY) agar at 19°C. Agar with mycelia was then aseptically cut into 1 × 1 〖"cm" 〗^"2" squares and placed into a Petri dish with 30 mL GY broth. After 2 days, the agar remnants were removed, and the growing mycelia were cut and washed repeatedly in sterilized tap water (TW) and then transferred into 20 mL fresh sterilized TW and kept for 18–24 h at 19°C (Kitancharoen and Hatai 1996). After the zoospores of the tested Saprolegnia strains were harvested; they were counted with the zoospore suspension was counted using a haemocytometer (Bürker Türk) and then added to experimental tanks at a concentration of 2 × 〖"10" 〗^"4" zoospore/l.