Eight zoosporic fungi viz., Achlya debaryana, A. flagellata, A. klebsiana, Aphanomyces laevis, Saprolegnia diclina, S. ferax, S. parasitica, and Pythium sp. were isolated from a large number of adult fishes of the species Mastacembelus armatus, Mystus vitatus, Nandus nandus, Tor putitora and T. tor of Nanak Sagar reservoir in Naini Tal district, India. Species of the parasites and the hosts were different in their pathogenicity and immunity, respectively. However, A. flagellata and S. parasitica appeared to be the most virulent. The severity of mycosis was primarily correlated to moderate water temperatures of 22-25 degrees C. High temperature (> 28 degrees C) retarded the disease process. The experimental inoculation with all the associated fungal species on Puntius conchonius in the laboratory produced clinical signs similar to the ones seen on infected fish in the reservoir. This is the first report on fish mycosis in the large reservoir located in the foot hill of Kumaun Himalaya.

Achyla debaryana (Saprolegniales, Oomycetes) is being reported for the first time as a fish pathogen, causing an epizootic of mycosis in a catfish, Mastacembelus armatus, in Nanak Sugar, a huge artificial reservoir and recognized fish production centre in Naini Tal district, Uttar Pradesh, India. The incidence of infection was found to be influenced by the physicochemical characteristics of water. The maximum disease severity (52 and 47.4%) occurred in March with moderate water temperature (22-24 degrees C), high dissolved oxygen content (9.5 mg l-1) and pH 8.6, while the lowest level of infection (3 and 1.8%) was recorded at high temperature (29-32 degrees C) during June.