Introduction to Fungi, Third Edition

434 HYMENOASCOMYCETES: HELOTIALES (INOPERCULATE DISCOMYCETES)
although resistance is not usually due to major
genes and is only partial. The underlying principle
in runner bean (Phaseolus vulgaris) seems
to be an enhanced tolerance of oxalic acid or a
restriction of its diffusion through the infected
tissue (Tu, 1985).
Sclerotia
Sclerotia of S. sclerotiorum form readily in culture
and have been the subject of investigations into
the physiology of their development (Willetts &
Wong, 1980; Willetts & Bullock, 1992). They are
of the terminal type. General aspects of sclerotial
development have been summarized on
pp. 18 21. The regulation of sclerotium formation
is interesting because it, too, is stimulated
by acid pH, and the Pac1 transcription factor is
involved (Rollins, 2003). Another signal known to
be a trigger of sclerotium development is
oxidative stress, e.g. lipid peroxidation or irradiation
with light. Excessive oxidation is prevented
by the synthesis of antioxidants such as b-
carotene or ascorbic acid (vitamin C), and if
high concentrations of these are added to
cultures of S. sclerotiorum, sclerotium formation
is inhibited (Georgiou & Petropoulou, 2001;
Georgiou et al., 2001).
15.2.4 Sclerotium cepivorum
This fungus causes white rot, the most serious
disease, of Allium spp., especially onions and
garlic. Sclerotia germinate by emitting hyphae
which grow towards the roots of the host plant
and cause necrotrophic infections with maceration
of the root and bulb base tissue. Large
quantities of various pectinolytic enzymes are
secreted (Metcalf & Wilson, 1999). New sclerotia
are formed on and in the decaying bulb tissues.
Sclerotia of S. cepivorum can survive in the soil for
many years or even decades (Coley-Smith, 1959),
and as little as one sclerotium per kg of soil can
cause serious disease losses (Crowe et al., 1980).
Apart from soil fumigation, no effective treatment
of infections or contaminated soil is
available (but see below), and fields may need to
be abandoned for Allium cultivation once
S. cepivorum has become established.
The root exudates of Allium spp. have long
been known to trigger germination of sclerotia
of S. cepivorum. Substances such as alkyl-cysteine
sulphoxides are themselves inactive but are
probably metabolized by soil microbes to release
volatile compounds which act as the stimulants
(Coley-Smith & King, 1969; King & Coley-Smith,
1969). One such substance, which is also produced
directly by Allium spp., is diallyl disulphide
(Storsberg et al., 2003). If this is sprayed onto an
infested field, it will trigger the germination of
sclerotia which is followed by their death if no
host plants are present. This idea seems to hold
potential for the control of S. cepivorum (Coley-
Smith, 1986; Coley-Smith & Parfitt, 1986). Garlic
powder worked into the soil seems to have
similar effects, most probably because of the
release of volatile substances from the nonvolatile
water-soluble alkyl cysteine sulphonates,
catalysed by soil bacteria (Fig. 15.4). Biological
control using Trichoderma spp., which secrete
chitinases capable of lysing hyphae
of S. cepivorum, may also be possible (Metcalf &
Wilson, 2001).
15.2.5 The life cycle of Botryotinia
(Sclerotinia) fuckeliana,anamorph
Botrytis cinerea
Because the apothecia of B. fuckeliana are not
commonly seen, the fungus is better known
by its macroconidial state, Botrytis cinerea. This
is ubiquitous on all kinds of moribund plant
material and is also associated with a wide range
of diseases often referred to as grey mould.
The name Botrytis cinerea is now known to be a
collective name used to describe a number of
closely similar, but genetically distinct, strains
(Giraud et al., 1999; Beever & Weeds, 2004). For
this reason some authors prefer to write of
a Botrytis of the cinerea type. In-depth treatments
of the biology of Botrytis cinerea have been
compiled by Coley-Smith et al. (1980) and Elad
et al. (2004). In addition to B. cinerea, there are
some 20 other Botrytis spp. causing diseases on
a wide range of host plants (Staats et al., 2005).
Macroconidia of B. cinerea are formed on
infected host tissue from dark-coloured branched
conidiophores. The tips of the branches are