In vitro screening of novel treatments for Aphanomyces invadans

Forty-nine compounds were tested in vitro fungicidal activity against hypha e of the fish-pathogenic Oomycete fungus, Aphanomyces invadans. These compr ised: (a) chemicals with previous reported activity against Oomycete fungi; (b) chemicals in use in Asia to treat ulcerative disease outbreaks; (c) co mmercial biocides and fungicides; (d) natural products with potential antim icrobial 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. in vadans hyphae at recommended treatment rates. A range of compounds that dem onstrated activity against the mycelium were selected for further testing i n a zoospore motility assay. Lower treatment concentrations were required t o inhibit zoospore motility than were required to inhibit hyphal growth. Zo ospore activity ceased within Ih 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, t ea tree (Melaleuca alternifloria) 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 m edium. Selected compounds were further tested for ability to inhibit zoospo re production by A. invadans mycelium over a 72-h period. In toxicity trial s, silver barb, Barbodes gonionotus (Bleeker), exposed to 2.5 p,p.m. coconu t diethanolamide; 2.5 p.p.m. OP10: 0.5 p.p.m. E-Z-Mulse(TM); 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-Mu lse(TM) suffered no mortalities and no obvious behavioural changes. Similar ly, rainbow trout, Oncorhynchus mykiss (Walbaum), exposed to 25 p.p.m. prop olis + 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 obv ious behavioural changes.