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Fusariumwilt-suppressive soils

The terms “long-life” and “short-life” for Fusarium
wilt-suppressive soils have now been replaced by
the terms disease-suppressiveand disease-conducive
soils. Many examples of disease-suppressive soils have
now been reported for vascular wilt diseases and
other pathogens across the world (Schneider 1982).
In almost all cases there is clear evidence that micro-
organisms contribute to suppressiveness, because sup-
pressive soils can be made conducive by pasteurization,
and conversely the introduction of a relatively small
amount of suppressive soil (10% or less) to pasteur-
ized soil or even to normal field soil can make a soil
suppressive.
Some of the most interesting examples of disease sup-
pression are found in sites such as the Chateaurenard
region of France, where vegetables have been grown
for centuries with little or no disease. In these sites
there is strong evidence that nonpathogenic strains
of Fusarium oxysporumcompete with the pathogenic
strains for organic substrates. Also, in these suppressive
soils the chlamydospores germinate poorly, the germ-
tubes grow poorly, the pathogen population in the soil
declines more rapidly, and a much higher inoculum
level is needed to cause disease. These soils are found
to be suppressive to a wide range of formae speciales
of F. oxysporum, but are not suppressive to other, unre-
lated pathogens such as Verticillium dahliae. Attempts

are now being made to exploit nonpathogenic strains

of F. oxysporumas commercial biological control agents

of the pathogenic fusarium wilt fungi. This is not

without its dangers, because we still know little about

the factors that govern the host ranges – or the patho-

genicity – of vascular wilt fusaria (Fravel et al. 2003).

The enigma of vascular wilt fusaria

The vascular wilt fusaria are enigmatic because, on the

one hand, they cause some of the most devastating

plant diseases and show a very high degree of host-

specificity, yet on the other hand they grow only in

the nonliving xylem tissues (after entry through the

immature root tips or through wounds) and only

invade the living plant tissues when the plant begins

to senesce. In effect, these fungi behave as weak para-

sites which, for most of their life in the plant, depend

on the very low levels of organic nutrients that leak

into the xylem vessels. In some respects this behavior

is similar to the mode of growth of the nonpathogenic

fungal endophyteswhich grow sparsely in the wall

spaces and intercellular fluids of plants without

causing obvious harm to the host (discussed below).

Endophytism is characterized by a basic compatibility

between the colonizing fungus and the host cells.

Consistent with this, some of the interactions between

vascular wilt fusaria and their host plants – for example,

F. oxysporumf. sp. pision peas – are governed by a

296 CHAPTER 14

Fig. 14.16Sections through the base of a diseased banana plant. (a) Heavily discolored vascular strands (single
arrowheads) where the fungus entered the base of the corm from roots, and (double arrowhead) where the infection
is spreading up the vascular strands in the leaf sheaths. (b) An older corm producing “suckers” (S) that will produce
the next banana plant. Vascular disease (V) is already spreading into the new plant.

(a) (b)