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