Introduction to Fungi, Third Edition

SCLEROTINIACEAE
431
Fig15.1 Sclerotinia
curreyana.(a)T.S.
sclerotium. Note the
stellate pith cells of
the host, Juncus effusus.
(b) T.S. sclerotium
showing
an ascogonium.
(c) Ascus and
ascospores.
(d) Microconidia in
culture. (e) T.S.
spermodochidium on
Juncus effusus.Note
the cavity lined by
phialides.
(f) Microconidia
from host.
state. All of these species can produce apothecia.
This is also true of Sclerotinia sclerotiorum, which
is considered to represent Sclerotinia sensu stricto
(Kohn, 1979). Sclerotium cepivorum produces neither
functional conidia nor apothecia, and the
sclerotia function purely as vegetative propagules,
germinating by hyphal growth. Microconidia
are sometimes produced by germinating
sclerotia, but these do not appear to have any
function. The relationship of S. cepivorum with the
Sclerotiniaceae has been deduced from DNAbased
studies (Carbone & Kohn, 1993). Whereas
macroconidia generally germinate readily and
play important roles in the spread of diseases,
microconidia may or may not germinate and
are considered to function mainly as spermatia,
i.e. agents of fertilization in sexual reproduction.
15.2.1 Sclerotinia curreyana and S. tuberosa
The apothecia of Sclerotinia (Myriosclerotinia)
curreyana, a pathogen of the rush Juncus effusus,
are common in May. They arise from black sclerotia
in the pith at the base of the Juncus stem
(Plates 7a,b). Infected stems look paler than
healthy stems, and by feeling down to the base
of an infected stem the sclerotium can be felt
as a swelling between finger and thumb.
The sclerotium has an outer layer of dark cells
and a pink interior which includes some of the
stellate pith cells of the host (Fig. 15.1a; Plate 7a).
One or several apothecia may grow from a single
sclerotium. The ascospores are released in late
spring and infect the new season’s stems. In
culture, germinated ascospores form a mycelium
which produces microconidia from small phialides
(Fig. 15.1d). Similar clusters of microconidia
can be found on infected Juncus later in
the season (Fig. 15.1f) where they line cavities
beneath the epidermis in the upper part of
infected culms. Whetzel (1946) has used the
term spermodochidium for these microconidial
fructifications (Fig. 15.1e). It is probable that
microconidia play a role in fertilization.
The apothecia of S. (Dumontinia) tuberosa
(Fig. 15.2) are about 2 cm in diameter and arise
from sclerotia within rhizomes of Anemone
nemorosa (Pepin, 1980). They may also occur on
garden Anemone where they are associated with
black rot disease. Microconidia are formed in
culture. Electron microscopy studies of the ascus
wall show that it has a two-layered wall, but the
two layers do not separate from each other,
i.e. the ascus is non-fissitunicate. The ascus apex
contains a thickened dome of wall material with
a central canal. As the ascus explodes, the apical
apparatus is everted (Verkley, 1993).