Introduction to Fungi, Third Edition

146 CHYTRIDIOMYCOTA
some of the species should be classified elsewhere.
Typical species are holocarpic. Some are
parasitic on fungi and aquatic plants or algae,
or saprotrophic on pollen (Sparrow, 1960).
Others parasitize rotifers (Glockling, 1998),
nematodes and their eggs (Tribe, 1977; Barron
& Szijarto, 1986), moss protonemata or leaves
and roots of higher plants (Macfarlane, 1968;
Johnson, 1969). Olpidium bornovanus (¼ O. radicale)
develops on various monocotyledonous and
dicotyledonous plant roots following inoculation
(Lange & Insunza, 1977). Olpidium brassicae is
common on the roots of cabbages, especially
when growing in wet soils, and is also found on
a wide range of unrelated hosts, but some
host specialization has been reported. Both
O. bornovanus and O. brassicae are vectors of a
number of plant viruses (Barr, 1988; Adams,
1991; Hiruki, 1994; Campbell, 1996) and this
topic is discussed more fully below. Weber and
Webster (2000a) have given practical details of
how to grow O. brassicae for observation on
Brassica seedlings. A film featuring O. brassicae is
also available (Webster, 2006a).
Epidermal cells and root hairs of infected
cabbage roots contain one or more spherical or
cylindrical thalli, sometimes filling the whole
cell (Fig. 6.14a). The cytoplasm of the thallus
is granular and the entire contents divide into
numerous posteriorly uniflagellate zoospores
that escape through one or more discharge
tubes which penetrate the outer wall of the
host cell (Temmink & Campbell, 1968). Release of
the zoospores takes place within a few minutes
of washing the roots free from soil. The tip of
the discharge tube breaks down and zoospores
rush out and swim actively in the water. The
zoospores are very small, tadpole-like, with
Fig 6.14 Olpidium brassicae in cabbage
roots. (a) Two ripe sporangia and one
empty sporangium in an epidermal cell.
Each sporangium has a single exit tube.
(b) Empty sporangium showing three exit
tubes. (c) Zoospores. (d) Zoospore cysts
on a root hair. Note that some cysts are
uninucleate and some are binucleate.
(e) Resting sporangia. (a,b,d,e) to same
scale.