Introduction to Fungi, Third Edition

SAPROLEGNIALES
79
prepared the way for the then revolutionary
theory that fungal infections can be the cause
rather than the consequence of disease, whereas
the latter stimulated research into chemical
control of diseases which directly gave rise to
the first fungicide, Bordeaux mixture (p. 119;
Large, 1940).
Although all members of Oomycota depend
on moist conditions for the dispersal of their
zoospores, they are cosmopolitan and ubiquitous
even in terrestrial situations. In species adapted
to drier habitats, the sporangia often germinate
directly to produce a germ tube, with zoospores
released as an alternative germination method
only in the presence of moisture, or lacking
altogether. Oomycota occur in freshwater, the
sea, in the soil and on above-ground plant
organs. Most are obligate aerobes, although
some tolerate anaerobic conditions (Emerson &
Natvig, 1981; Voglmayr et al., 1999), and one
species (Aqualinderella fermentans) is obligately
anaerobic and lacks mitochondria (Emerson &
Weston, 1967). Oomycota live either saprotrophically
on organic material, or they may be
obligate (biotrophic) or facultative (necrotrophic)
parasites of plants. Some can also cause diseases
of animals, such as Aphanomyces astaci which has
all but eliminated European crayfish from many
rivers (p. 94), Saprolegnia spp. which cause serious
infections of farmed fish, especially salmon
(Plate 2a; Dick, 2003), or Pythium insidiosum
causing equine phycomycosis (de Cock et al.,
1987). Yet other Oomycota, notably Lagenidium
giganteum, parasitize insects and may prove
valuable in the biological control of mosquito
larvae (Dick, 1998).
5.1.5 Classification
As indicated above, the classification of
Oomycota at the level below the phylum is still
an ongoing process, and it is difficult at present
to reconcile the different classification schemes
that are being proposed. Kirk et al. (2001) listed
eight orders in the phylum Oomycota, of which
Dick (2001b) treated six within the class
Peronosporomycetes, his equivalent to the
Oomycota, considering the other two of
uncertain affinity (incertae sedes). These groups
are summarized in Table 5.1.
5.2 Saprolegniales
The order Saprolegniales is currently divided
up into two families, the Saprolegniaceae
(e.g. Achlya, Brevilegnia, Dictyuchus, Saprolegnia,
Thraustotheca) and Leptolegniaceae (Aphanomyces,
Leptolegnia, Plectospira), totalling 132 species in
about 20 genera (Dick, 2001a; Kirk et al., 2001).
The Saprolegniales are the best-known group of
aquatic fungi, often termed the water moulds.
Members of this group are abundant in wet soils,
lake margins and freshwater, mainly as saprotrophs
on plant and animal debris. Whilst some
Saprolegniales occur in brackish water, most
are intolerant of it and thrive best in freshwater.
A few species of Saprolegnia and Achlya are economically
important as parasites of fish and
their eggs (Willoughby, 1994). Aphanomyces
euteiches causes a root rot of peas and some
other plants, whilst A. astaci is a serious parasite
of the European crayfish Astacus (Alderman et al.,
1990). Algae, fungi, rotifers and copepods may
also be parasitized by members of the group, and
occasional epidemics of disease among zooplankton
have been reported.
Members of the Saprolegniales are characterized
by coarse, stiff hyphae which branch
to produce a typically fast-growing mycelium.
The hyphae of Saprolegniales are coenocytic,
containing a peripheral layer of cytoplasm
surrounding a continuous central vacuole.
Cytoplasmic streaming is readily observed in
the peripheral cytoplasm. Numerous nuclei are
present. Mitotic division is associated with the
replication of paired centrioles and the development
of an intranuclear mitotic spindle; the
nuclear membrane remains intact throughout
division (Dick, 1995). Filamentous mitochondria
and lipid droplets can also be observed in
vegetative hyphae. The mitochondria are orientated
parallel to the long axis of the hypha and
are sufficiently large to be seen in cytoplasmic
streaming in living material. Important physiological
work has been carried out on the