Introduction to Fungi, Third Edition

HYPOCREALES
343
Fig12.18 Canker caused by Nectria galligena
on the twig of an old apple tree.
readily distinguished from conidial pustules by
their bright red colour and their granular
appearance. The pustules are regarded as perithecial
stromata, bearing as many as 30 perithecia.
Ripe perithecia contain numerous
club-shaped asci, each with eight two-celled
hyaline ascospores (Fig. 12.20c). These are somewhat
unusual in being multinucleate (El-Ani,
1971). There is no obvious apical apparatus to the
ascus (Strickmann & Chadefaud, 1961).
Perithecial development begins by the formation
of ascogonial primordia beneath the surface
of the conidial pustule. The details of development
are as described on p. 337. An important
feature is the development of apical paraphyses
which grow downwards from the upper part of
the perithecial cavity. As the asci develop from
ascogenous hyphae lining the base of the
perithecial cavity, they grow upwards through
the mass of apical paraphyses, which are difficult
to find in mature perithecia (Strickmann &
Chadefaud, 1961).
Other species of Nectria differ from N. cinnabarina
in a number of ways. In many, the perithecia
are not grouped together on a stroma, but
occur singly (e.g. in N. galligena). The asci of some
species, e.g. N. mammoidea, have a well-defined
apical apparatus (Fig. 12.21a).
12.4.3 Fusarium (Nectriaceae)
Fusarium-type conidia are known in several
species of Nectria and also in the related genus
Gibberella (Samuels et al., 2001). For many
Fusarium species a teleomorph has not yet been
found. The number of species recognized by
morphological characters in pure culture varies
from one authority to another. Booth (1971)
included 44 species and 7 varieties; Gerlach and
Nirenberg (1982) included 73 species and 26
varieties, while Nelson et al. (1983) distinguished
30 species. However, using molecular techniques
a large number of species which cannot be
distinguished by morphological characters are
now being defined (O’Donnell, 1996), and a
publically accessible database of diagnostic DNA
sequences has been established as an aid to identification
(Geiser et al., 2004). Species of Fusarium
are cosmopolitan in soils from the permafrost of
the Arctic to sand in the Sahara desert (Booth,
1971). They are particularly common in cultivated
soil and are associated with a wide range of
plant diseases as indicated in Table 12.3 (Nelson
et al., 1981). Some species are also known to
cause diseases of humans such as onychomycosis
(nail infections), keratomycosis of the cornea,
ulcers, necroses, skin infections and fatal infections
of internal organs, especially in immunocompromised
patients (Joffe, 1986; de Hoog et al.,
2000a).
A characteristic feature in the asexual reproduction
of Fusarium is the development from
phialides of fusoid, transversely septate macroconidia
with a basal contracted foot cell
(Fig. 12.22b). Microconidia, also formed from
phialides, develop in some species. On plant
hosts macroconidia often accumulate in
orange pink sporodochia (Plate 5e, Fig. 12.22a).
Macroconidia may be dispersed by rain splash