Introduction to Fungi, Third Edition

HYPOCREALES
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Fig12.22 Fusarium culmorum. (a) Part of a sporodochium with clusters of phialides producing macroconidia. (b) Mature
macroconidium from a two-week-old culture.The foot cell is indicated by an arrowhead. (c) Older macroconidia (eight-week-old)
in which some cells have collapsed and the others have turned into chlamydospores. (d) Chlamydospores formed by a hypha
submerged in agar. (b d) to same scale.
F. oxysporum, and this has been summarized by Di
Pietro et al. (2003).
The control of diseases caused by Fusarium is
mainly through the use of resistant host plant
cultivars (Beckman, 1987). The disastrous economic
consequences of the outbreak and spread
of Panama wilt of banana caused by F. oxysporum
f. sp. cubense which attacked the popular banana
variety ‘Gros Michel’ were overcome by switching
production to clones of the resistant variety
‘Cavendish’ which could be planted into soils
heavily contaminated with F. oxysporum f. sp.
cubense (Moore et al., 2001). Another wellcharacterized
example is tomato wilt caused by
F. oxysporum f. sp. lycopersici. Resistance seems to
be based mainly on major-gene resistance, and
numerous resistance genes have been crossed
into the cultivated tomato plant from wild
relatives. These occur in several discrete gene
clusters (Sela-Buurlage et al., 2001). A range of
tomato cultivars with resistance against one or
more of the three races of F. oxysporum f. sp.
lycopersici is available for planting.
Another approach actively pursued at present
is the biological control of F. oxysporum. An
interesting strategy is the use of apathogenic
strains of F. oxysporum which colonize the root
cortex but not the vascular system, and therefore
have an endophytic lifestyle. They inhibit infections
by pathogenic strains. Various explanations
have been offered, including competition for
nutrients, the induction of systemic acquired
resistance in plants pre-infected with the apathogenic
strains, and direct competition between
apathogenic and pathogenic strains in the root
cortex (Larkin & Fravel, 1999; Bao & Lasarovits,
2001). Other soil micro-organisms such as
Pseudomonas spp. are also being developed as
biocontrol agents.
Mycotoxins
Some species of Fusarium (e.g. F. graminearum and
F. moniliforme) are seed-borne, with mycelium
being present inside the grain and often producing
macroconidia on the surface. Infected seed
not only ensures carry-over of the disease to
following seasons but can be a source of
mycotoxins if fed to livestock or consumed by
humans. Mycotoxins such as zearalenone, fumonisin,
trichothecenes and vomitoxin (Fig. 12.23)