I - --ii

Table 2
Reactions to ascochyta blight for bilked lines and
segregating. generations sown during 1980-81 at Tel Hadya.
Bulked lines/ Reaction to ascochyta blight on 1-9 scale Total
segregating
generations 1 2 3 4 5 6 7 6 9
Bulked line 0 0 5 11 20 6 3 I 0 46
F 6 progeny 0 0 24 0 42 8 72 49 195 390
F 5 progeny 0 0 2 0 5 3 39 51 94 194
F 4 progeny 0 0 344 0 322 121 181 26 27 1021
vanced in the off-season site at Terbol. The F2 segregating populations are raised
in the disease nursery at Tel Hadya, where mass selection is practiced for blight
resistance. The resistant plants are bulk harvested and large seeds are selected in
the laboratory, bulked and grown in the F3 generation in off-season at Terbol,
where selection for less photoperiod-sensitive types is exercised because the day
length is about 14 hours at the start of the growing season and it decreases as the
season advances. These plants having longer day length requirements do not
mature and are naturally eliminated. The mature plants are bulk harvested. It is
envisaged that this practice will continue until F6 when single plants will be
selected, yield tested and elite lines made available to cooperators for evaluation
under their local conditions and for possible release as commercial cultivars.
Breeding for Stable Resistance
Breeding for Horizontal Resistance
van ier Plank (1968) developed the concept of horizontal resistance which is'a
race-nonspecific or broad-based or field resistance, and follows the concept of the
multiline as proposed by Borlaug (1959). According to the former author, when a
variety is more resistant to some races of a pathogen than others, the resistance is
called "vertical" or "perpendicular". When resistance is evenly spread against
all races of the pathogen it is called "horizontal" or "lateral". He further explained
that vertical resistance implies a differential interaction between varieties
of the host and races of the pathogen; in horizontal resistance there is no
differential interaction. He proposed that in breeding for horizontal resistance
such as field resistance, vertical resistance can be prevented from interfering in
two ways; first, by excluding vertical resistance genes, and second, by exposing
genotypes under test to a race of the pathogen virulent enough to match any
vertical resistance genes that may be present.
101