Aspects of durable resistance in wheat to yellow rust

In Kenya, the number of virulence factors of the yellow rust populations showed a considerable increase and a wide variability. Selecting for complete to near complete resistance to yellow rust and other cereal rust diseases, was followed by a rapid erosion of resistance.Partial resistance in wheat to yellow rust appeared to be difficult to select for. However, quantitatively resistant (QR) genotypes with a reduced disease severity (DS) and intermediate infection types (IT) were frequently observed. In addition, a fair number of wheat cultivars seemed to be durably resistant (DR) as their resistance was effective even after more than 25 years.DS and area under disease progress curve proved to be suitable parameters to measure QR while IT was too unstable a trait to rely on. QR can assessed reliably in small plots since there was no interplot interference observed. During screening and selection for QR, the level of nitrogen (N), heading date, observation date and leaf position are factors that should be considered by the breeder. DS and IT increased moderately with increased Nitrogen level. The magnitude of such increase was not the same for all genotypes.Selection for QR can be best carried out by discarding the most susceptible and most resistant plants or lines. In the field QR genotypes different significantly in DS and IT while race specificity to some yellow rust races and ineffective seedling resistance genes to race 134E150 were observed. By assessing the stability over environment's of a range of genotypes, genotype x location interaction occurred fairly often for QR while the durably resistant cultivars showed by far the least interactions with location indicating high environmental stability for DR.The resistance in five genotypes with different levels of QR seems to be based on rather few factors, one for Pr-2 to some three for Pr-7. The five genotypes seem to have factors in common or different factors that are strongly linked. The resistance in five durably resistant cultivars seemed to be based predominantly on a complex of strongly linked factors that inherited almost as a single factor in crosses with the highly susceptible Morocco.