Agriculture Reference
In-Depth Information
disease susceptibility. Since inception of the survey there has been a sequence of
often devastating epidemics of this disease as a succession of new pathotypes
developed to overcome a previously resistant combination of genes. Bayles
et al.
(1997) discuss these interactions, using data from the UKCPVS for both yellow rust
on wheat and powdery mildew on barley. In both cases, the complexity of pathogen
races reduces the flexibility of conventional breeding techniques to provide
resistance. The changes illustrate the ability of pathogens to overcome the best
endeavours of plant breeders who have exploited most of the readily available
resistance for these diseases.
One of the UKCPVS's most important products has been diversification schemes
(Priestley, 1981) developed to help farmers to identify the risk of disease. Cultivars
are assigned to individual diversification groups (e.g., Anon., 1997a) selected on the
basis of genetic resistance to pathogens and the frequency of matching virulences in
the pathogen population. Cultivar selection, with the aim of reducing disease
pressure, is an illustration of the difficulty of disease control in sustainable systems.
The cultivation of suitable mixtures of cultivars, chosen from different diversifi-
cation groups, can reduce disease. However, in most areas of western Europe,
pathogen races are now so complex that it is difficult to combine appropriate resis-
tance gene combinations with suitable agronomic or market quality requirements.
The additional complication that it is often easier to market pure stands of single
cultivars also reduces the uptake of this approach; it is far easier to meet the grain
specifications of different customers by mixing discrete lots of known cultivars as
necessary. Wolfe
et al.
(1981) discussed the benefits of cultivar mixtures which is
also considered in more detail by Finckh and Wolfe in Chapter 10 of this topic. The
chosen mixture should comprise of cultivars with different sources of resistance,
though this does not necessarily guarantee greater yield stability than that provided
by pure stands (Akanda and Mundt, 1997).
An interesting manifestation of crop sensitivity to disease occurred in maize during
the 1970s, when male sterile lines were being used for seed production.
One strain of the common disease southern corn leaf blight (caused by
Helminthosporium
maydis
) became especially virulent on hybrids bred using the Texas form of cytoplasmic
male sterility. The disease was effectively controlled by reverting to mechanical detasseling
of the male parents.
Despite such difficulties as those outlined above, effective use of genetic
resistance to specific diseases or tolerance of disease remain essential elements of
sustainable agriculture A range of plant breeding approaches is now being explored
throughout the world to improve the robustness of crop plant resistance. These
involve the use of molecular markers to identify and more precisely to characterise
resistance genes, and the technique commonly known as genetic modification; this
offers the prospect of durable disease resistance with associated opportunities
to reduce significantly or even obviate the need for fungicide treatment. This would
be of substantial benefit to sustainable and organic systems. However, genetic
modification involving 'genetic engineering' techniques to transfer genes between
unrelated species would, at present, be unacceptable to the organic movement.
Knowledge of the background resistance of potential cultivars provides the basic
understanding necessary for the design of crop rotations and fungicide programmes
