Agriculture Reference
In-Depth Information
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by the high levels of disease found in each of Peacocke s (1995) trials where, in
certain cases, trials were planted on land following lengthy fallow periods of up to
25 years.
Although in Peacocke's trials the observed rates of disease progress differed
significantly among cultivars, none showed complete immunity to infection. It is
possible that genotypes carrying high levels of monogenic, race-specific resistance
identified in earlier pathogenicity studies were absent from the breeding programme,
or overlooked in the selection process. In the case of both Sima and Kuyuma, initial
crosses had identified several genotypes showing immunity to anthracnose infection.
As a result, it was apparent that despite high disease pressure in Peacocke's trials, in
which cryptic error may have caused difficulties in the identification of resistance
genotypes (
sensu
Van der Plank, 1963), durable rate-reducing resistance was
observed during the trials.
In certain circumstances, horizontal resistance can delay the start of an epidemic.
Since horizontal resistance reduces the percentage of spores that successfully infect,
the first cycle of a polycyclic epidemic could be delayed in the same way as vertical
resistance delays the start of an epidemic. Alternatively, by reducing the general
level of disease, horizontal resistance may lower the level of initial inoculum surviv-
ing the winter to start a new disease cycle. Horizontal resistance may also reduce the
progeny/parent ratio to 1 or less, this being below the level required for disease to
increase.
5.3 VERTICAL RESISTANCE
Vertical resistance operates against a specific genetic component of the pathogen
species but not against all and it is equated with the gene-for-gene concept of Flor
(1955). Vertical resistance usually involves resistance mechanisms inherited mono-
genically or oligogenically. Such genes are matched, at least potentially, by corre-
sponding genes for pathogenicity within the parasite population. Vertical resistance
may be applied to both complete resistance and the components of incomplete
resistance that interact differently with components of the pathogen population
(Johnson, 1984). It has generally been considered that vertical resistance serves to
reduce the infection frequency of inoculum, thereby blocking or delaying the poten-
tial epidemic (Van der Plank, 1963).
The advantages of vertical resistance to the breeder are that it is often easily
identifiable, manipulated and incorporated into cultivars as a result of its simple
inheritance and high levels of resistance. However, in selecting for such complete
resistance, the genetic basis of resistance is narrowed, since other forms of resistance
are hidden and may be lost in the selection process. Van der Plank (1963) termed this
the 'vertifolia' effect. Typically, selection in the pathogen population has eventually
led to the emergence of a pathotype virulent with respect to the genetic, vertical
resistance of the host. This cycle of resistance identification, incorporation and
breakdown has been termed the 'boom and bust' cycle (Priestley, 1978). Although
examples of durable
vertical resistance forms have been reported, durability may be
influenced by the size of the pathogen population, the environment in which the host
