Water moulds (oomycetes) of the order Saprolegniales, such as Saprolegnia and Aphanomyces
species, are responsible for devastating infections on fish in aquaculture, fish farms and
hobby fish tanks. Members of the genus Saprolegnia cause Saprolegniosis, a disease that is
characterised by visible white or grey patches of filamentous mycelium on the body or fins
of freshwater fish. Up till 2002, Saprolegnia infections in aquaculture were kept under control
with malachite green, an organic dye that is very efficient at killing the pathogen. However,
the use of malachite green has been banned worldwide due to its carcinogenic and
toxicological effects and this has resulted in a dramatic re-emergence of Saprolegnia infections
in aquaculture. As a consequence Saprolegnia parasitica is now, economically, a very
important fish pathogen, especially on catfish, salmon and trout species, and warrants further
investigation to develop new alternative control strategies.