The cause of deeply penetrating ulcers of Atlantic menhaden Brevoortia tyrannus has been the subject of significant research efforts in recent years. These lesions and the associated syndrome termed ulcerative mycosis have been observed along the East Coast of the United States since at least the early 1980s. Although Aphanomyces spp. were isolated from these lesions in the mid to late 1980s, similar lesions could not be reproduced by experimental infections of Atlantic menhaden with these isolates. The identical characteristic histologic appearance of granulomatous inflammation surrounding the penetrating fungal hyphae occurs in fish with epizootic ulcerative syndrome (EUS), as reported throughout South Asia, Japan, and Australia. Aphanomyces invadans has been found to be the causative agent of EUS in all of these countries. Using methods developed for the study of EUS, we successfully isolated an organism for which the DNA sequence, morphology, temperature and salinity growth characteristics, and infectivity of chevron snakehead Channa striata are identical to A. invadans. Using the polymerase chain reaction assay for A. invadans, we were able to demonstrate the presence of the organism from Atlantic menhaden lesions collected in U.S. estuarine waters from Delaware to South Carolina. In addition, the organism was present in lesions on a bluegill Lepomis macrochirus from a farm pond in Georgia and channel catfish Ictalurus punctatus from a farm pond in Louisiana.

Forty-nine compounds were tested in vitro for fungicidal activity against hyphae of the ®sh-pathogenic Oomycete fungus, Aphanomyces invadans. These comprised: (a) chemicals with previous reported activity against Oomycete fungi; (b) chemicals in use in Asia to treat ulcerative disease outbreaks; (c) commercial biocides and fungicides; (d) natural products with potential antimicrobial activity; and (e) surfactants tested separately and in combination with some of the above treatments. No compounds tested proved as effective as malachite green, but some low-toxicity natural compounds and particular surfactants showed potential for further studies. Some compounds that are currently in use in Asian aquaculture were shown to have no effect on A. invadans hyphae at recommended treatment rates. A range of compounds that demonstrated activity against the mycelium were selected for further testing in a zoospore motility assay. Lower treatment concentrations were required to inhibit zoospore motility than were required to inhibit hyphal growth. Zoospore activity ceased within 1 h of exposure to 2.5 parts per million (p.p.m.) coconut diethanolamide; 1.25 p.p.m. propolis + 0.5 p.p.m. 13/6.5; 5 p.p.m. neem (Azadirachta siamensis) seed extract + 0.01 p.p.m. OP10; 20 p.p.m. tea tree (Melaleuca alterni¯oria) oil; and 25 p.p.m. D-limonene + 0.05 p.p.m. E-Z-Mulse TM . The treated spores were shown to be non-viable in culture medium. Selected compounds were further tested for ability to inhibit zoospore production by A. invadans mycelium over a 72-h period. In toxicity trials, silver barb, Barbodes gonionotus (Bleeker), exposed to 2.

Aphanomyces invadans (Saprolegniaceae) is a peronosporomycete fungus associated with the serious fish disease, epizootic ulcerative syndrome (EUS), also known as mycotic granulomatosis. In this study, interspecific relationships were examined between A. invadans isolates and other aquatic animal pathogenic Saprolegniaceae, and saprophytic Saprolegniaceae from EUS-affected areas. Restriction fragment length polymorphisms and sequences of ribosomal DNA confirmed that A. invadans is distinct from all other species studied. A sequence from the internal transcribed spacer region ITS1, unique to A. invadans, was used to design primers for a PCR-based diagnostic test. Intraspecific relationships were also examined by random amplification of polymorphic DNA using 20 isolates of A. invadans from six countries. The isolates showed a high degree of genetic homogeneity using 14 random ten-mer primers. This provides evidence that the fungus has spread across Asia in one relatively rapid episode, which is consistent with reports of outbreaks of EUS. Physiological distinctions between A. invadans and other Aphanomyces species based on a data set of 16 growth parameters showed remarkable taxonomic congruence with the molecular phylogeny.