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Keywords:

  • Aphanomyces invadans;
  • Oomycete fungus;
  • treatment;
  • epizootic ulcerative syndrome

Abstract

Forty-nine compounds were tested in vitro for fungicidal activity against hyphae of the fish-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 diethanol- amide; 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 alternifloria) oil; and 25 p.p.m. D-limonene + 0.05 p.p.m. E-Z-MulseTM. 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 2.5 p.p.m. coconut diethanolamide; 2.5 p.p.m. OP10; 0.5 p.p.m. E-Z-MulseTM; 20 p.p.m. neem seed extract + 0.5 p.p.m. OP10; and 5 p.p.m. D-limonene + 0.5 p.p.m. E-Z-MulseTM suffered no mortalities and no obvious behavioural changes. Similarly, rainbow trout, Oncorhynchus mykiss (Walbaum), exposed to 25 p.p.m. propolis + 1 p.p.m. OP10; 10 p.p.m. neem seed extract + 0.01 p.p.m. 13/6.5; and 10 p.p.m. D-limonene + 0.01 p.p.m. OP10 suffered no mortalities and no obvious behavioural changes.