8th INTERNATIONAL WHEAT CONFERENCE

The 2 nd generation slow-rusters had maximum DSS ranging from 16 to 48%, with an
average of 26%, intermediate between that of the major-gene genotypes (most with immune
reaction with few showing maximum DSS of 5-20%, with an average of 3%) and
that of the 1 st generation slow-rusters (30-60% with an average of 48%). Susceptible genotypes
had a maximum DSS ranging from 45% to 100%, averaging 78%. The best four
genotypes belonging to the 2 nd generation slow-rusters category did not have a maximum
DSS above 23% in any of the environments, and over all environments their maximum
DSS was 16-18%. These values for the 2 nd generation slow-rusters, as well as those of the
corresponding AUDPC, were significantly lower than the maximum DSS and AUDPC of
any of the parent genotypes or of the other unrelated 1 st generation slow-rusters. In fact,
they were lower than, or similar to, the maximum DSS and AUDPC values observed for
some of the resistant genotypes carrying known major genes such as LrCamayo present
in the breeding line Camayo and the cultivar CIRNO C2008. These results indicate that
the intercrossing of 1 st generation slow-rusters that carry exclusively minor genes and
subsequent selection for minimum disease levels resulted in the identification of lines
with enhanced leaf rust resistance through the accumulation of different minor genes
with additive effects. The levels of resistance obtained so far are promising and clearly approaching
those observed for some major gene based resistance. Whereas this provides a
strong “proof of concept” for the utilization of minor-gene based resistance to leaf rust in
durum wheat, the challenge remains to identify and characterize additional sources of diverse
minor genes, likely from germplasm groups unrelated to the CIMMYT germplasm,
and use them towards the production of genotypes with close-to-immune reaction to
the pathogen, allowing levels of disease well below those that would cause any yield loss
under a wide range of environments.
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