Agriculture Reference
In-Depth Information
that in fields in which the straw from preceding wheat crops was regularly
incorporated,
Oculimacula
infection of subsequent crops was less than in fields in
which the straw was burnt. The burning reduced the number of
Oculimacula
individuals surviving from the crop. However, it also reduced the amount of straw
incorporated and so reduced the population of lignin- and cellulose-decomposing
saprophytic fungi. In fields with straw incorporated, the straw on which the
overseasoning
Oculimacula
survived was rapidly colonised by specialist
decomposers and the
Oculimacula
was deprived of nutrients, so that fewer spores
were produced than in the burnt fields.
7.8.5 Co-evolution of host and pathogen
The linkage between host and pathogen populations sets up a co-evolutionary race
between pathogen and host, the host evolving towards resistance and the pathogen to
virulence. In agriculture this can produce a boom and bust cycle of repeated release
and failure of cultivars (Barrett, 1988). In natural settings also, the consequences of
this evolution may be far from a smooth progression (Gilbert, 2002). For example,
in the
Linum
case (Burdon, 1993), the rust can multiply dramatically within a season
and therefore the genetic structure of the population may adapt very quickly to that
of the host. The host, however, has a seed-bank. This will be well-stocked with the
survivors of past years of severe disease, to which the pathogen may now be ill-
adapted. This is likely to produce extremely complicated population dynamics but
no models have yet been published.
In agricultural settings, the host population is linked to the pathogen population
through farmers' and breeders' responses to disease. Much of the detail in the
evolutionary and dynamical response of a pathogen population to the host
composition depends on chance events (Shaw, 1994a). For example, in the mid
1980s barley cv. Triumph was widely grown in the UK, carrying effective resistance
alleles to
Blumeria graminis
. After virulence against these alleles became common,
almost half the population of
B. graminis
was represented by a single phenotype
carrying at least three virulence alleles not needed on any variety then grown widely
(Brown and Wolfe, 1990). This is almost certainly because the necessary mutations
for virulence on cv. Triumph occurred in a single individual already carrying the
extra virulences. The result was an abundant pathogen population, composed of a
few very common clones and many much rarer ones (Brown
et al.
, 1990).
In populations of aerially dispersed pathogens, once a virulence allele is present
at a moderate frequency its fate depends on how common the matching resistance is,
and on the strength of selection against the virulence over the whole life-cycle of the
pathogen. Many virulence alleles appear to carry moderately small selective
disadvantages in the absence of the corresponding resistance, and will persist at
moderate frequencies for many years. If the matching host resistance is
reintroduced, or introduced into a new area, the population of the pathogen carrying
the matching virulence allele is likely to become common faster than when the
cultivar was first introduced, because the allele is already present in the population.
