Agriculture Reference
In-Depth Information
with age and the manner in which it may be obscured by vertical resistance.
Furthermore, the disparity shown by breeding lines when components of resistance
are compared indicates the severe limitations raised when initial screening is
conducted under controlled conditions, especially given the sensitivity of compo-
nents to environmental fluctuations. However, a controlled-environment screening
system may have considerable advantages over a full-scale field test in terms of time
and manpower cost effectiveness.
Although horizontal resistance may be inherited either polygenically or
oligogenically, the former is far more common (Robinson, 1969). In a number of
cases, a small degree of race specificity has been demonstrated - for example, in
potatoes without resistance to potato blight (R0 varieties) when challenged by indi-
vidual isolates of
Phytophthora infestans
(Caten, 1974). However, Caten found no
evidence for increasing performance by
P. infestans
isolates on individual potato
varieties and, in general, reductions in the level of expression of horizontal resis-
tance occur rarely and are quantitative and partial (Bennett, 1984). Van der Plank
(1971) illustrated the stability of R0 potato varieties in their resistance to late blight
by referring to varieties that had appeared on an annual list of reactions to blight
continuously between 1938 and 1968 (Table 5.3). In terms of foliar resistance, the
variety that changed most was Record, which fell from equal top in 1938 to sixth in
1968. Over the same time period, the ranking for the tuber resistance of Record
showed a change from second to first place (Van der Plank, 1971) and it is unlikely
that a new race of
P. infestans
arose which was capable of causing greater damage to
foliage whilst at the same time being less well adapted to attack the tubers.
Table 5.3. Stability of horizontal resistance as illustrated by the field response of 10 potato
varieties to infection by
Phytophthora infestans
in 1938 and 1968 (from Van der Plank, 1971)
Cultivar
Resistance level
1938
1968
Alpha
8
7
Bevelander
8
7
Furore
8
7
Record
8
6
Noordeling
7
7
Voran
7
7
Ultimus
6
5
Eigenheimer
4
5
Bintje
3
3
Eersteling
3
3
The effect of lengthening the latent period, reducing the rate of lesion expansion and
lowering the numbers of spores produced on each lesion is to elongate the disease
progress curve horizontally, as classically shown by Van der Plank (1968) using the
example of potato varieties without R genes for resistance to late blight. Disease
