Septoria and Stagonospora Diseases of Cereals - CIMMYT ...

Session 6C: Breeding for Disease Resistance
Breeding for Resistance to the Septoria/Stagonospora
Blights of Wheat
M. van Ginkel and S. Rajaram
Wheat Program, CIMMYT, El Batan, Mexico
Abstract
Genetic resistance remains the first line of defense against the septoria foliar blights, especially in developing countries.
Resistance genes with major or minor effects may be either recessive or dominant. A few genes may be enough to confer
resistance, as in the case of partial resistance. Additive gene effects contribute more to resistance than dominance effects.
Among normally maturing semidwarf wheats possessing resistance, those carrying Rht2 may be more resistant to Septoria
tritici than those with Rht1. In the case of Stagonospora nodorum, resistance of the flag leaf and of the spike may be at least
partly under separate genetic control.
Five confounding factors complicate selection for resistance: 1) maturity and plant height affect the expression of
resistance; 2) the relationship between seedling and adult plant responses is highly inconsistent; 3) the correlation between
disease response and yield loss is very variable; 4) there is interaction among fungal isolates on the leaf surface; and 5) it is
essential to determine the implications of the existence of races for resistance breeding.
Durability of resistance across years and locations is a particular concern, and examples of both erosion of resistance and
stable resistance have been put forward. There is clear proof of differential variety by isolate interactions at the seedling stage
and on adult plants in the field. However, almost without exception, the size of the interaction component is about a
magnitude smaller than that of the main effects due to varieties and isolates. Recent molecular experiments on the genetic
structure of S. tritici populations have found a lack of adaptation to the host genotype.
The methodology applied at CIMMYT for pyramiding resistance genes in wheat includes utilizing proven resistance
sources as parents, selecting for resistance using a shuttle breeding program between contrasting locations, and confirming
resistance through global multilocation testing. The best resulting lines are fed back into the crossing program as parents. The
first genepool exploited for resistance to S. tritici by CIMMYT breeders originated in Brazil and Argentina, followed by
winter wheats and, subsequently, Chinese materials. New sources of resistance, such as highly promising synthetic wheats,
are currently being exploited in combination with the previous sources.
Globally the septoria diseases
have increased in importance over
the past decades, and as Walter
Spurgeon Beach wrote in 1919:
“The genus is the more worthy of
study on account of its high
economic importance.” The two
main septoria blights addressed in
this paper are caused by Septoria
tritici and Stagonospora nodorum.
However, inferences can be drawn
for the septoria blights in other
cereals, such as Septoria passerinii on
barley. Several excellent overviews
have been written on the genetics of
resistance to the septoria diseases of
cereals and on how to breed for
such resistance (Shipton et al., 1971;
King et al., 1983; Nelson and
Marshall, 1990; Lucas et al., 1999;
Eyal, 1999). Most of those reviews
were published 10 years or more
ago, but a great deal of the
information they contain is still
correct and relevant today.
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This paper emphasizes some of
the more recent literature (post 1990)
on the genetics and breeding of
resistance, and how these new
insights may be applied in developing
varieties that express durable
resistance to the septoria blights.