Factors influencing the sporulation and cyst formation of Aphanomyces invadans, etiological agent of ulcerative mycosis in Atlantic menhaden, Brevoortia tyrannus

Oomycete infections caused by Aphano- myces invadans occur in freshwater and estuarine fishes around the world. Along the east coast of the USA, skin ulcers caused by A. invadans are prevalent in Atlantic menhaden, Brevoortia tyrannus. From lab- oratory observations low salinities appear crucial to transmission of the pathogen. To better understand aspects of transmission, we characterized sporulation and cyst formation of secondary zoospores of two iso- lates of A. invadans at different salinities and tem- peratures. Sporulation occurred only at low salinities. At room temperature (ca. 20-22 C), using ''pond wa- ter'' augmented with artificial sea salts, the endemic strain WIC and the Thailand strain PA7 of A. inva- dans produced free-swimming secondary zoospores at salinities of 0, 1 and 2 psu (practical salinity unit 5 ‰), but not at 4 psu or higher. Secondary zoo- spores of another species, ATCC-62427 (Aphanomyces sp.), were observed at 1, 2, 4 and 8 psu but not at 0 and 12 psu. Secondary zoospores of all three isolates, especially WIC, were abundant and motile 1-2 d post- sporulation. Sporulation was temperature dependent and occurred over a relatively narrow range. No spor- ulation occurred at 4, 30 or 35 C for either WIC or PA7. For both strains zoospore production within 1- 3 d after the initiation of sporulation was more pro- lific at 25 C than at 20 and 15 C. At 15 C production of zoospores was sustained over 11 d for WIC and 5 d for PA7. At room temperature single WIC second- ary zoospores remained motile 12-18 h. Salinities ex-