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Robinson (1976) has further developed the concept of horizontal resistance
stating that agriculturally horizontal resistance is permanent resistance. Its use
does not lead to a "boom-and-bust cycle" of cultivar production, and breeding for
horizontal resistance should be cumulative; a good cultivar being replaced by a
better cultivar.Horizontal resistance provides an incomplete but permanent disease
control. Polygenically inherited resistance is always horizontal but not all
horizontal resistance is inherited polygenically. Horizontal resistance and horizontal
parasitic ability are independent of each other and according to Robinson
(1976) there is no need to search for a source of resistance. Even within the most
susceptible cultivars it is possible to develop horizontal resistance utilising random
cross-pollination in a genetically flexible population combined with selection
pressure exerted by the parasite, which should occur as naturally as possible.
This screening should lead to an increase in all variable resistance mechanisms
against all locally important parasites; for this reason there should be a bteeding
program for each ecological zone. Some 10-15 host generations are necessary in a
population of some thousands of individuals. Robinson (1976) has cited a successful
example of breeding maize cultivar resistance to Pucciniapolysora in
East Africa; although it was accidental rather than arising out of a well planned
experiment.
Nelson (1978), who never accepted the term horizontal resistance in the sense
proposed by van der Plank (1968), stated that he did not believe that there were
special vertical genes, but that there were just genes for resistance, some of which
had major effects in some situations or genetic backgrounds, and the same genes
in other situations or backgrounds could have less effect or contribute to partial
or rate reducing form of resistance. He believes that many major plant diseases
can be managed effectively by returning the plants and their parasites to genetic
equilibrium. He suggested the development of cultivars with many vertical
resistance genes (genetic pyramiding), so that even if the pathogen is able to
develop the necessary virulence, the residual reaction of the resistance genes
confers a high degree of partial resistance, so that a disease epiphytotic will no'
occur.
Although there is divergence of views on breeding for horizontal resistance,
such a program has been initiated at ICARDA. Four crosses, involving susceptible
parents with a rating of 7, were made in 1979-80 and the Fis were grown
during the 1980 off-season. A large number of F2 plants were grown in the 1980­
81 season in the disease nursery and their reactions to blight are given in Tab. 3.
A number of plants in the segregating populations were more resistant than
either of the parents involved in the crosses. In future, a group of moderately
resistant plants will be crossed with another group of similar plants selected
within each cross. Also a group of moderately resistant plants in one segregating
population will be crossed with a similar group of plants in another population.
The FIs will be advanced in the off-season and the F2 populations grown in the