Strategies for the utilization of partial resistance for the control of cereal rusts

In cereals all resistance to cereal rusts is of the species-specific type, i.e. the resistance is effective to one rust species only. Against each rust pathogen two types of species-specific resistance can be recognized: i) A major gene, hypersensitive type of resistance characterized by low infection types, race-specificity and lack of durability; ii) a quantitative type of resistance (partial resistance), characterized by a reduced rate of epidemic build-up despite a high, susceptible infection type, by absence of large race-specific effects and by durability. In the absence of major genes, selection for partial resistance is easy. Even a mild selection against susceptibility, if applied consistently, is highly effective in accumulating genes for partial resistance. This mild selection enables the breeder to select for other characteristics at the same time. If one wishes to increase partial resistance in the presence of major genes that have not been fully neutralized by the pathogen, the efficiency of selection is considerably less. If possible, the breeder should expose the host population to a single race of the pathotype, a race that neutralizes a maximum number of major genes. In this host population, the breeder should remove the most susceptible genotypes at each stage of selection and also those genotypes that show a low infection type. If it is too difficult to score infection types reliably, the breeder should remove the most resistant genotypes together with the most susceptible ones as the former are assumed to carry major genes. In some situations, the pathogen population to which the host is exposed cannot be controlled and exists as a mixture of races. Selection for partial resistance is very difficult in this case. Continuous removal of the most susceptible lines together with those lines that are nearly unaffected will tend to favor partial resistance, but the progress may be slower than hoped for