Introduction to Fungi, Third Edition

SPIZELLOMYCETALES
147
Fig 6.15 Olpidium brassicae.
Diagrammatic representation of
L.S. of zoospore (afterTemmink &
Campbell,1969a).
a spherical head and a long trailing flagellum.
The fine structure of the zoospore is summarized
in Fig 6.15. A distinctive feature is the banded
rhizoplast which connects the kinetosome to
the nucleus (Temmink & Campbell, 1969a;
Lange & Olson, 1976a,b; Barr & Hartmann,
1977). This structure has also been reported
from the zoospore of the eucarpic chytrid
Rhizophlyctis rosea (see p. 148; Barr & Hartmann,
1977).
The zoospores swim actively in water for
about 20 min. If roots of cabbage seedlings are
placed in a suspension of zoospores, these settle
on the root hairs and epidermal cells, withdraw
their flagella and encyst. The cysts are attached
by a slime-like adhesive (Temmink & Campbell,
1969b). The cyst wall and the root cell wall at
the point of attachment are dissolved and
the root cell is penetrated. The cyst contents
are transferred to the inside of the host cell,
probably by the enlargement of a vacuole which
develops inside the cyst, whilst the empty cyst
remains attached to the outside. The process
of penetration can take place in less than
one hour (Aist & Israel, 1977). Within 2 days of
infection, small spherical thalli can be seen in
the root hairs and epidermal cells of the root,
carried around the cell by cytoplasmic streaming.
The thalli enlarge and become multinucleate.
Within 4 5 days discharge tubes develop
and the thalli are ready to release zoospores.
In some infected roots, stellate bodies with
thick folded walls, lacking discharge tubes,
are also found (Fig. 6.14e). These are resting
sporangia. There is no evidence that they are
formed as a result of sexual fusion either in
O. brassicae or in O. bornovanus (Barr, 1988).
Although biflagellate zoospores may occur in
O. brassicae, these possibly result from incomplete
cleavage (Temmink & Campbell, 1968) and
zoospores with as many as 6 flagella have been
observed (Garrett & Tomlinson, 1967). The resting
sporangia are capable of germination
7 10 days after they mature, and germinate by
the formation of one or two exit papillae
through which the zoospores escape.
Virus transmission by Olpidium
Several plant viruses are transmitted by zoospores
of Olpidium. By analogy with plant virus
transmission by aphids, Adams (1991) arbitrarily