Introduction to Fungi, Third Edition

SCHIZOSACCHAROMYCETALES
253
about 5 a.m. and the spores appear mature by
8 a.m. However, maximum spore discharge does
not occur until about 8 p.m. Outside the ascus,
ascospores or yeast cells may continue budding
and the fungus can be grown saprotrophically
as a yeast in agar or liquid culture. The yeast
cells are often pigmented due to the presence of
b-carotene and several other carotenoid pigments
(van Eijik & Roeymans, 1982). Young
leaves can be infected from such yeast cells and
it has been shown that a culture derived from
a single ascospore can cause infection resulting
in the formation of a fresh crop of asci, so that
T. deformans is homothallic. In this respect it differs
from some other species, e.g. T. epiphylla,
where the fusion of yeast cells, presumably of
different mating types, is necessary before infection
can occur (Kramer, 1987). In T. deformans the
binucleate condition is established at the first
nuclear division of a yeast cell placed on a peach
leaf, and the two daughter nuclei remain associated
in the germ tube which penetrates the
cuticle (Fig. 9.2j). In other Taphrina species, the
germ tube penetrates through stomata but is
unable to breach the intact cuticle (Taylor &
Birdwell, 2000).
9.2.2 Growth hormones
In infections of peach leaves with T. deformans,
the distortions of the host tissue are associated
with division and hypertrophy of the cells of the
palisade mesophyll. In liquid cultures, especially
on media containing tryptophane, considerable
quantities of the auxin-type phytohormone
indole acetic acid (IAA) have been demonstrated.
A number of different cytokinins are also
produced by several species of Taphrina in culture
(Kern & Naef-Roth, 1975; Tudzynski, 1997).
Together, these hormones promote processes of
cell division, enlargement and differentiation in
plants, and leaves infected with T. deformans
show higher levels of auxins and cytokinins
than uninfected leaves (Sziráki et al., 1975). It is
therefore tempting to assume that the fungus
produces these substances also in planta.
However, this has not been formally proven
yet, and the Taphrina peach system seems to
have been less thoroughly examined than the
interaction between Plasmodiophora brassicae and
cabbage plants (see p. 63).
9.2.3 Control of Taphrina deformans
Taphrina deformans is by far the most serious
pathogen among the Taphrinales, and it occurs
wherever peach or almond trees grow. It is not
yet entirely clear how T. deformans overwinters;
Butler and Jones (1949) and Smith et al. (1988)
considered it unlikely that the mycelial form is
involved because leaves harbouring mycelium
are shed in the autumn. It is more probable
that yeast cells arising from discharged ascospores
survive saprotrophically on the surface of
twigs or in bud scales. Between November and
March, the yeast cells develop thick walls and in
spring, as the peach buds open, they produce
germ tubes which penetrate the young leaves.
The first symptoms of infection can be seen as
soon as the buds break, but no further infection
occurs from about early July onwards. This may
be because T. deformans has a relatively low
temperature maximum of 26 30°C (Butler &
Jones, 1949).
Good chemical control of T. deformans can
be achieved by spraying with Bordeaux mixture
in autumn after leaf fall, in order to reduce the
population of yeast cells on the twigs. Another
spray in early spring, at the time of bud swelling,
will give improved control of infection because
the time span in which Taphrina can infect
is limited. Dithiocarbamates or other simple
fungicides are commonly used in spring
(Smith et al., 1988). Other diseases caused by
Taphrina can be controlled in a similar way if
necessary.
9.3 Schizosaccharomycetales
The classification of the Schizosaccharomycetales
has been the subject of controversial
discussions, but the emerging consensus is
that there is only one genus with three species,
S. japonicus, S. octosporus and S. pombe (Kurtzman
& Robnett, 1998; Vaughan-Martini & Martini,
1998a; Barnett et al., 2000). All three species grow