Introduction to Fungi, Third Edition

432 HYMENOASCOMYCETES: HELOTIALES (INOPERCULATE DISCOMYCETES)
Fig15.2 Apothecia of Sclerotinia tuberosa rising from
subterranean sclerotia formed on rhizomes of Anemone
nemorosa.
15.2.2 Monilinia fructigena and M. laxa
Monilinia fructigena is the cause of a brown fruit
rot of apples, pears, plums and other stone fruits
(Byrde & Willetts, 1977). Although the apothecial
state is only rarely formed, the disease is
common and is transmitted by means of conidia.
Apples and pears showing brown rot bear buffcoloured
pustules of conidia often in concentric
zones (Fig. 15.3a). Sporulation is stimulated by
light, and adjacent zones correspond to daily
periods of illumination. The conidia are blastoconidia
formed in chains which extend in length
at their apices by budding of the terminal conidium.
Occasionally more than one bud is formed,
and this results in branched chains (Fig. 15.3b).
Conidiogenesis of this type is characteristic
of the anamorphic genus Monilia. Infection of
the fruit is commonly through wounds caused
mechanically or by insects such as codling
moth, wasps or earwigs (Croxall et al., 1951;
Xu & Robinson, 2000). Fruits left lying on the
ground are the source of infection in the following
season. During the winter, infected fruits
become mummified, and the shrivelled fruit
thoroughly colonized by mycelium is interpreted
as the sclerotium. In the following year the
sclerotium may produce further conidial pustules.
Infections can develop as a post-harvest
disease in stored apples, and in some varieties
a twig infection (spur canker) may also occur.
A similar group of diseases of apple and
plum is caused by Monilinia laxa which also has
a Monilia conidial state. In addition to fruit rot,
this species causes blossom and shoot blight,
in which infected fresh shoots wilt and become
coated by conidial pustules. Monilinia fructicola
causes brown rot especially of peaches and nectarines
in North and South America, South Africa,
Australia and the Far East, but has not been
reported from Europe. It produces the apothecial
state more readily than the other species (Holtz
et al., 1998), and ascospores released from overwintered
mummified fruits can be a source of
inoculum in the field (Tate & Wood, 2000). These
and a fourth species of the brown fruit rot
complex, M. polystroma, can be distinguished by
means of morphological features and DNA
sequences (van Leeuwen et al., 2002).
15.2.3 Sclerotinia sclerotiorum
This species causes a range of diseases (Sclerotinia
rot, white mould, stalk break) in over 400
cultivated and wild plant species belonging to
some 75 different families (Boland & Hall, 1994).
The most important crop plants affected are
sunflower, soybean and oilseed rape, with crop
losses approaching 100% under conditions favourable
to the disease. Sclerotia are formed on
decaying crop debris and remain viable in a
dormant state in the soil for many years, especially
if deep-ploughed. Sclerotia located within
the top 3 cm of soil germinate to produce hyphae
or apothecia in spring. Plants can be infected
either from mycelium or from ascospores; there is
no macroconidial state. Phialidic microconidia
are formed and probably serve as spermatia.
Infection by S. sclerotiorum is often initiated by
a saprotrophic phase on dead leaves or petals
during which mycelial biomass is generated,
prior to the attack on the living plant tissue.
Above-ground shoots and, to a lesser extent, roots
can be infected, and infections of living tissues
are strongly necrotrophic. This necrotrophic
phase is followed by further saprotrophic
growth and the formation of sclerotia. The
infection cycle in S. sclerotiorum is therefore
tri-phasic. Reviews of the general biology and
pathology of S. sclerotiorum have been written by