Agriculture Reference
In-Depth Information
excretion of antibiotics (interference competition) or, indirectly, by triggering
changes in their hosts, or supporting larger populations of hyperparasites than would
otherwise exist; (the example at the end of section 7.8.3 can be seen as a competitive
one: in the presence of a population of the stem-infecting rust, the leaf-infecting rust
has a smaller population).
The degree to which two species compete is determined by the similarity with
which they use available resources. Two species that use resources identically will
not be able to co-exist indefinitely in a stable environment. One will inevitably be
very slightly more efficient and will grow more rapidly than the other, supplanting it
slowly or rapidly. This is the competitive exclusion principle (Begon
et al.
, 1996).
At first sight, this conclusion is incompatible with the regular occurrence, on the
same plant organ, of many similar pathogens: for example, in the UK on wheat, it is
easy to find leaves simultaneously infected with
Mycosphaerella graminicola
,
Didymella exitialis
and
Phaeosphaeria nodorum
. In fact, several of the assumptions
underlying the competitive exclusion principle are violated: hosts vary dramatically
in abundance and quality during the year, so that the environment for a plant
pathogen is never stable; pathogens and hosts are spatially aggregated, so that the
strength of competition between species is reduced relative to that between related
individuals within a dense patch of disease and there may well be subtle differences
in resource use by apparently similar pathogen individuals, related to temperature
and wetness requirements for infection, preferred age of host tissue, etc. On
the other hand, the apparent replacement on banana of
Mycosphaerella musicola
by
the more pathogenic
Mycosphaerella fijiensis
(cause of sigatoka disease) in parts of
the world where both have occurred (Mourichon and Fullerton, 1990) is in accord
with the theory.
It seems common for competition between plant pathogens to be very asymmetric.
For example,
Phaeosphaeria nodorum
reduces the growth rate of
Blumeria graminis
populations on the same leaves but preferentially infects leaves infected with
B.
graminis
(Weber
et al.
, 1994). Similar predisposing effects are known in a number of
pathogen combinations but evidence on epidemic progress in both pathogens is
usually lacking (Brokenshire, 1974; Jones and Jenkins, 1978; Madariaga and
Scharen, 1986; Stahle and Kranz, 1984).
An interesting case where competition has been revealed by agricultural change
is the balance among the eyespot pathogens
Oculimacula yallundae
and
O. acuformis
and the sharp eyespot pathogen
Rhizoctonia cerealis
. Treatment of
north European wheat crops with demethylation-inhibiting fungicides has had two
effects. First, an increase in population density of
R. cerealis
correlates well in
individual fields with a decrease in
Oculimacula
spp. and this is due to antagonism
(Kapoor and Hoffmann, 1984). Second,
O. acuformis
increases relative to
O.
yallundae
(Bateman
et al.
, 1995). This is more likely to be because of differential
sensitivity to fungicide treatment than competitive release because the absolute
numbers of
O. acuformis
do not necessarily increase and
O. acuformis
appears to
multiply later in the season than
O. yallundae
(Goulds and Fitt, 1988), so it may
partly escape the effects of spring fungicides.
It also appears that populations of
Oculimacula
spp. are partly controlled by
competition with true saprophytes (Jalaluddin and Jenkyn, 1996). The evidence is
