Agriculture Reference
In-Depth Information
Wolfe and McDermott (1994) conclude that 're-cycling' of alleles in
B. graminis
has 'little prospect except in the short-term in limited locations'.
Under exceptional circumstances, the long-distance movement characteristic of
aerially dispersed pathogens may make selection for virulence ineffective. For
example, in the Canadian great plains wheat cv. Selkirk remained resistant to
Puccinia graminis
(cause of black stem rust) for many years. It contained a suite of
at least six resistance alleles, to all of which individually virulence was present
within North America. However, the rust did not overwinter in the area where
Selkirk was grown, but migrated in each season from the south. The pathogen
population probably did not evolve resistance because the precise combination of
resistance alleles in Selkirk was never used in the hosts of the overwintering
population (Wolfe and Knott, 1982).
With soil-borne pathogens of restricted mobility, the genetic structure of a
population is likely to be much more fragmented, and rapid replacement of the
whole genetic structure of the population is less likely. Use of single resistance
genes against some soil-borne pathogens has been successful in some cases, and
even where the pathogen has evolved to overcome the resistance in one place this is
followed by sporadic rather than rapid and universal spatial spread.
There is increasing evidence that populations of many trees are limited by the
build-up of root pathogens in the soil around a mature tree (Augspurger, 1983; Hood
et al.
, 2004; Packer and Clay, 2004). Most seedlings fall close to the parent tree and
are killed by root damage from soil pathogens which have multiplied on the parent
roots, without killing it because of its size and reserves. Only rare individual
seedlings germinating far from the parent survive. This seems likely to produce a
particularly fine-grained mosaic of host and pathogen co-adaptation, since selection
on the pathogen around one parent tree will be long-continued, but surviving
seedlings nearby are likely to select for different strains.
7.9 SPATIAL POPULATION STRUCTURE
As section 7.8.5 showed, the spatial structure of populations can have a profound
influence on population dynamics. It can also make the study of dynamics much
harder, because the population studied may, in fact, be composed of several
independent populations which are being unknowingly averaged, or the population
may be part of a much larger system and be controlled by factors which cannot be
observed within the area studied.
In some pathosystems, vectors or propagules can move long distances (see also
Chapter 6) (Brown and Hovmøller, 2002). Examples include rusts and powdery
mildews of cereals or many whitefly- and aphid-borne viruses (Irwin and Thresh,
1988). In such cases it is not sensible to consider a crop area isolated unless it is a
long distance from other such crops; often, this may mean many kilometres. The
dynamics of the pathogen within a defined crop area will, therefore, be determined
both by processes within the crop and by immigration and emigration of propagules
from the surrounding crops. This can make experiments on control very difficult,
because the small crop areas desirable to make it feasible to include many treatments
