Agriculture Reference
In-Depth Information
CHAPTER 5
EPIDEMIOLOGICAL CONSEQUENCES OF PLANT DISEASE
RESISTANCE
M.L. DEADMAN
5.1 INTRODUCTION
The concept of resistance is, or should be, central to any plant disease
management programme. Other disease control practices, including the use of
chemical intervention techniques, cultural control methods and biological control,
can all be used to minimize crop damage. Each of them, however, can be seen
essentially as a complement to plant disease resistance. For farmers and growers,
plant resistance offers the most cost-effective front line method for disease
control, since its adoption requires little alteration to existing practices. This is
especially important for the so-called resource-poor producers - those without the
financial or technological wherewithal to adopt chemical or other high cost or high
input strategies.
Resistance refers to the ability of the host plant to overcome, either completely
or in part, the effect of a pathogen. The scale of this ability may vary from small,
where there may only be a slight suppression of disease development, to large,
where pathogenesis is incomplete. Incomplete pathogenesis can adequately
suppress disease. If the effects are large enough, pathogen reproduction rates are
slowed to the extent that the pathogen population merely replaces lost individuals
but fails to increase in size. If the resistance effects are small, disease increases
more or less rapidly and, as a consequence, other control techniques will be
required. However, the use of resistance is not without its own problems. Genetic
uniformity within a plant population can lead to pressure for new virulences to
develop within the pathogen population, or minor pathogens to become
increasingly important in some agricultural production systems. When identical
plants are sown over large areas there are no spatial or temporal obstacles to
epidemic progress.
Resistance in plants can take two possible, though not mutually exclusive, forms;
Van der Plank (1963) termed these horizontal and vertical resistance. Horizontal
resistance is expressed against all races of the pathogen, i.e. it is race, pathotype or
biotype non-specific. Vertical, race-specific or specific resistance can be described
as resistance that is race or biotype specific. In addition, consideration needs to be
given to the concepts of induced resistance, non-host immunity and tolerance. Each
of these will be considered in turn in this chapter.
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