Agriculture Reference
In-Depth Information
is grown, the mechanisms by which resistance is conferred and its genetic back-
ground (Parlevliet, 1979). Such factors are generally beyond the control of plant
breeders.
The epidemiological effects of vertical resistance in its simplest form are clear.
When the initial inoculum of a pathogen population is heterogeneous for pathogenic
compatibilities, the plant s resistance will be effective against some of the races but
not against others. In other words, the amount of inoculum capable of causing
disease will have been vertically reduced. This results in a delay in the onset of
disease. Classic examples of vertical resistance are frequently taken from crop-
pathogen interactions where there is a well established gene-for-gene relationship.
The potato variety Pentland Dell has the
Rl, R2
and
R3
genes for resistance to
P
.
infestans.
In the UK, the variety was first released in the early 1960s at a time
when there were no compatible races of
P.
infestans.
As a consequence, no late
blight developed on the variety. During 1966, late blight was first recorded on this
variety but only in a limited number of fields. This suggests that the initial selection
of biotypes compatible with Pentland Dell occurred during or before the 1966
season. During the 1967 season race (1,2,3) was prevalent and the resistance within
Pentland Dell was ineffective over large parts of the country. In 1968, this race
became even more common and disease levels on Pentland Dell were similar to
those on other potato varieties. During 1966 and 1967, when race (1,2,3) was rare in
some potato fields, disease onset on Pentland Dell was delayed relative to that on
other varieties and also delayed relative to the time of first appearance of symptoms
on the same variety in 1968.
An illustration of the rate at which vertical resistance can be overcome is
provided by Parry (1990) using the example of the spring barley cultivar, Triumph.
A race of mildew with a virulence gene capable of overcoming the vertical
resistance present in Triumph was identified in 1977, yet it was not until 1983 that
the frequency of the gene had increased to the extent that major outbreaks of disease
occurred. The rating for resistance of Triumph fell from 8 in 1983, when the
popularity of the variety was at its peak, to 2 in 1985 (Table 5.5).
The resistance gene,
Mlg,
effective against powdery mildew, was the first major
gene deliberately introduced into barley. The gene remained effective against
mildew in Germany in the 1930s and 1940s while the area under cultivation
to the
variety containing the gene remained small. Beginning in the late 1940s the area
under cultivation increased rapidly. This was followed by an increase in the
frequency of the corresponding virulence gene,
Vg.
Eventually the
Mlg
varieties lost
their mildew resistance as the pathogenicity gene became increasingly common
(Wolfe, 1984).
The
Mlg
gene was widely used in European breeding programmes and conse-
quently the matching pathogenicity,
Vg,
also became widespread. This increase in
the frequency of
Vg
was partly through increased selection and partly through the
movement of spores from areas where it was already common. It was only during
the mid 1980s that there was a decline in the frequency of
Vg;
for over 50 years its
frequency remained high because of the large proportion of spring barley crops
possessing
Mlg
(Wolfe, 1984).
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