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.

English

9910|AGRIS 0001|R99-00CIMPU

Jose Juan Caballero

CIMMYT Publications Collection