Introduction to Fungi, Third Edition

PYTHIALES
97
Fig 5.15 Pythium mycelium in the rotting tissue of a cress
seedling hypocotyl. Note the spherical sporangium initial and
the absence of haustoria.
coenocytic, with typically granular cytoplasmic
contents (Fig. 5.15). At first there are no septa,
but later cross walls may cut off empty portions
of hyphae. Thick-walled chlamydospores may
also be formed. There are no haustoria.
Several species are known to cause damping
off, e.g. P. debaryanum and, perhaps more
frequently, P. ultimum. Pythium aphanidermatum
is associated with stem rot and damping off of
cucumber, and the fungus may also cause rotting
of mature cucumbers. Pythium mamillatum
causes damping off of mustard and beet seedlings
and is also associated with root rot in Viola.
Many Pythium spp. have a very wide host range;
e.g. P. ultimum parasitizes over 150 plant species
belonging to many different families (Middleton,
1943; Hendrix & Campbell, 1973). Far from
parasitizing only plant roots, several soil-borne
species, e.g. P. oligandrum, P. acanthicum and
P. nunn, are capable of attacking hyphae of
filamentous fungi, including plant-pathogenic
species and even other Pythium spp. (Foley &
Deacon, 1986b; Deacon et al., 1990). Attack may
be mediated by the secretion of wall-degrading
b-1,3-glucanase, chitinase and cellulase, or by
inducing the host to undergo autolysis (Elad
et al., 1985; Laing & Deacon, 1991; Fang & Tsao,
1995). In contrast to plant-pathogenic Pythium
spp., the mycoparasitic species require thiamine
for growth and are unable to utilize inorganic
nitrogen sources. These deficiencies may
explain their mycoparasitic habit (Foley &
Deacon, 1986a). Other species of Pythium parasitize
freshwater and marine algae (Kerwin et al.,
1992).
The taxonomy of Pythium is somewhat
confused at present due to the existence of
numerous synonyms. Including a few varieties,
Dick (2001a) listed 129 names in current use.
Since the morphological characteristics traditionally
used for diagnosis can be variable, the
delimitation of species and their assignment to
the genus Pythium will have to await the results
of detailed molecular phylogenetic analyses
which are in progress (Matsumoto et al., 1999;
Lévesque & de Cock, 2004). Keys and descriptions
have been published by Waterhouse (1967,
1968), van der Plaats-Niterink (1981) and Dick
(1990b).
Asexual reproduction
The mycelium within the host tissue or in
culture usually produces sporangia, but their
form varies. In some species, e.g. P. gracile,