Introduction to Fungi, Third Edition

436 HYMENOASCOMYCETES: HELOTIALES (INOPERCULATE DISCOMYCETES)
Fig15.5 Botrytiscinerea. (a) Conidiophores developing from a
sclerotium. (b) Apex of conidiophore showing origin of conidia
as blastospores. (c) Conidium germinating to produce phialides
and microconidia (after Brierley,1918).
bearing microconidia but no sclerotia, whilst the
other four produce mycelia bearing sclerotial
stromata. Apothecia develop on the strains forming
sclerotia if microconidia are transferred to
them. Thus the mating behaviour of S. narcissi
differs from that of B. fuckeliana and we can say
that S. narcissi is sexually dimorphic. This type of
behaviour is not common in ascomycetes and it
is possible that such an incompatibility system
has been derived from the more usual system
exemplified by B. fuckeliana by aberrations which
prevent the normal sequence of development of
sexual organs in basically hermaphrodite forms
(Raper, 1959).
A somewhat related phenomenon is found
in Sclerotinia trifoliorum, in which each ascus
contains ascospores of two different sizes. The
four large ascospores germinate to give rise to
homothallic (self-fertile) mycelia, whereas the
four smaller ascospores produce self-sterile
mycelia (Uhm & Fujii, 1983a,b). As discussed earlier
in detail for Saccharomyces cerevisiae (see
Fig. 10.5), the two mating type alleles of ascomycetes
differ strongly in the genes they encode
and are thus termed idiomorphs. Although no
detailed studies seem to have been carried out
on the Sclerotiniaceae, in other filamentous
ascomycetes such as Pyrenopeziza brassicae (see
p. 439), heterothallic strains carry either one or
the other of the two idiomorphs whereas in
homothallic species both are fused together
and expressed simultaneously. In Sclerotinia
trifoliorum, the formation of small ascospores
may be preceded by a unidirectional switch from
homothallic to heterothallic, i.e. the deletion of
one of the two mating types during meiosis
(Harrington & McNew, 1997).
Yet another kind of mating behaviour is seen
in Sclerotinia sclerotiorum, which is homothallic.
A single ascospore culture produces microconidia
and sclerotia which bear ascogonial coils
beneath the rind. The transfer of microconidia to
the sclerotia on the same mycelium results in
the formation of apothecia (Drayton & Groves,
1952). A similar process of self-fertilization also
occurs in Sclerotinia (Botryotinia) porri.
15.2.7 Pathogenicity of Botrytis cinerea
Botrytis cinerea is pathogenic on over 200 species
of plants. Serious diseases of crops are grey
mould of lettuce, tomato, strawberry and raspberry,
die-back of gooseberry and damping-off
of conifer seedlings. A special case is bunch rot
of grapes. Under normal circumstances, infected
grapes shrivel and ultimately fall to the ground,
forming mummies in which the fungus can
survive the winter. Mummies give rise to
infective macroconidia in the following spring.
This type of bunch rot causes serious
crop losses in both white and red grapes.
Under certain circumstances and with certain
grape varieties, however, Botrytis causes the
‘noble rot’ in which infections take a milder
course and allow the grape to dry out gently,
concentrating its sugar and flavours in the process.
The resulting wine is much sweeter and
richer than normal table wine and is consumed