Introduction to Fungi, Third Edition

HYPOCREALES
345
Fig12.21 Nectria mammoidea. (a) Ascus;
note the apical apparatus.
(b) Cylindrocarpon-type conidiophore with
phialides and conidia.
from such pustules (see Jenkinson & Parry,
1994). In culture, greasy accumulations of macroconidia
are termed pionnotes. Survival in soil is
probably in the form of dormant chlamydospores
which may be formed as a result of energy
deprivation or in response to bacterial secretions.
They are thick-walled and develop
by modification of segments of the vegetative
mycelium or by enlargement and modification
of a segment of a macroconidium (Schippers &
van Eck, 1981) (Figs. 12.22c,d). Chlamydospores
are induced to germinate in the presence of root
secretions (for references see Griffiths, 1974).
One of the most commonly isolated species
from soil is F. oxysporum, which often grows in
association with roots. Many isolates are nonpathogenic
but this fungus may also be a serious
plant pathogen, reported from a very wide range
of plant hosts. Although no sexual state is
known, molecular studies show that it is monophyletic
with the Gibberella Fusarium complex
and can be regarded as an asexual Gibberella
(Samuels et al., 2001). In this species over 80
formae speciales characteristic of different host
genera have been recognized, in addition to a
large number of vegetative compatibility types
and races (Gordon, 1993; Gordon & Martyn,
1997). The large amount of genetic variation in
F. oxysporum in the absence of conventional
sexual reproduction is possibly explained by
the demonstration of parasexual recombination
even between members of different vegetative
compatibility groups (Molnár et al., 1990).
Fusarium wilts
As can be seen from Table 12.3, formae speciales of
F. oxysporum cause wilt diseases of numerous crop
plants. The range of affected hosts is so wide that
it is easier to mention those plants unaffected by
Fusarium wilts, i.e. grasses and many tree species.
Although F. oxysporum has no known sexual state,
strains of this species complex show huge
genetic variation which may come about by the
presence of transposable elements (Daboussi &
Capy, 2003) and the parasexual cycle (Teunissen
et al., 2002). The variability of F. oxysporum is
such that a given disease, e.g. Panama wilt of
banana, is caused by several unrelated strains
(K. O’Donnell et al., 1998), and that strains with
different host specificities may be closely related
(Hua-Van et al., 2001).
Wilting is associated with the presence of
fungal hyphae in the xylem vessels. Fusarium
oxysporum can persist in the soil as chlamydospore
inoculum for many years and infects the
roots of hosts by direct penetration and