Introduction to Fungi, Third Edition

SCLEROTINIACEAE
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Fig15.4 Non-volatile water-soluble
Allium metabolites (left) and their
volatile breakdown products (right).
S-Methyl-L-cysteine sulphoxide (methiin)
is common in many plants, and its
sulphide breakdown products do not
trigger sclerotium germination in
Sclerotium cepivorum. Alliin
(S-2-propenyl-L-cysteine sulphoxide
¼ S-allyl-L-cysteine sulphoxide), isoalliin
and propiin are typical of members of
the genus Allium.Their volatilebreakdown
products, especially mercaptans,
sulphides and disulphides, are potent
triggers of sclerotium germination.
Diallyl disulphide is the major flavour
component of garlic.
thin-walled and bud out to form numerous
elliptical multinucleate conidia which are
blastospores (Hughes, 1953). These are easily
detached by the wind, or are thrown off as
the conidiophores twist hygroscopically
(Figs. 15.5a,b). Conidia can also be dispersed by
the fruitfly Drosophila melanogaster (Louis et al.,
1996) and other insect vectors. Uninucleate
microconidia are formed by clusters of phialides
which arise directly from the mycelium (Weber &
Webster, 2003) or from germinating macroconidia
(Fig. 15.5c). The microconidia have been
claimed to be capable of germination (Brierley,
1918) but do not do so in our experience. They
are probably mainly involved in sexual reproduction,
i.e. they function as spermatia. Sclerotia
are formed at the surface of infected tissues and
the fungus overwinters in this form. In spring
the sclerotia may develop to give rise to tufts
of macroconidia or, much less commonly, to
apothecia. One or several stalked apothecia may
arise from one sclerotium, with the stalk 1 cm
or more in length and the apothecial disc a few
millimeters in diameter. Botryotinia fuckeliana is
heterothallic with a bipolar mating system. In a
single-ascospore culture, macroconidia, microconidia
and sclerotia can be formed on the same
agar plate (Weber & Webster, 2003), but apothecia
never develop. However, apothecia will form
if microconidia of one mating type are applied
to sclerotia of the opposite mating type (Faretra
et al., 1988). Like the great majority of ascomycetes
(Bistis, 1998), B. fuckeliana therefore shows
physiological heterothallism. The life cycle of
this fungus is summarized in Fig. 15.6.
15.2.6 Other life cycles in Sclerotinia
A deviation from the typical ascomycete life
cycle of B. cinerea (Fig. 15.6) is found in Sclerotinia
(Stromatinia) narcissi (Drayton & Groves, 1952).
Of the eight spores formed in the asci of this
fungus, four germinate to produce mycelia