Genetic analysis of wheat-rust interactions and strategies to breed for durable resistance

Assessment of designated series of genes for resistance against Indian pathogen populations of all the three rusts and genetic basis for rust resistance in released cultivars have revealed that several of the effective genes, mostly derived from species related to T. aestiovum, remained unexploited. A number of these genes have been successfully introgressed into formerly popular wheat cultivars that provided easily usable diverse resistances for tailoring new generations of high yielding rust resistant cultivars. Some of these stocks when employed as parents in several cross combinations in a breeding programme have generated a number of promising cultivars with diverse resistances. Deployment of diverse resistances in a crop population has helped in suppressing the intensity, effectiveness and frequency of rust epiphytotics and thus served as a buffer in checking disease spread. It is well recognized that most resistances are eroded by the new pathogenic races soon after the cultivar is extensively grown. Evolution of new virulences in each of the pathogen populations as influenced by resistance genes deployed in the local cultivars has been described. Adult plant resistance (APR) is conferred by genes that are ineffective in seedlings but becomes operative in adult plants. They are implicated in durability but usually confer partial resistance. A number of cultivars both from national and international programmes and certain near-isogenic lines carrying specific leaf-rust resistance genes as possessing diverse sources of APR to leaf rust have been identified. A new source Federation* 4/Kavkaz carrying novel complementry genes, sperately derived from Federation and Kavkaz, determined adult plant resistance to leaf rust. Some of these sources, besides Lr34/Yr18, Sr2, Sr26 and Sr31 that are recognized to confer durable resistance, have potential to be durable. In certain cultivars Lr34 and Sr2 in combination with other specific genes have been successfully utilized for durable resistance. A novel adult plant resistance gene associated with durability to leaf rust in cv. Arjun has been identified. Combination of more than one resistance genes each conferring resistance to existing race spectrum in a cultivar is one strategic approach to achieve durable resistance. Identification of markers linked to different resistance genes for selection of plants with such genic combinations in a segregating population of resistance breeding is important. Molecular marker selection techniques that have become recently available are still emerging with tremendous potential to pyramid desired gene combinations. However, until the molecular marker selection techniques become routinely available, conventional means of pyramiding some highly effective genes in the background of adult durable incomplete resistance is a more practical approach of breeding durable and adequate levels of resistance. Durable resistances controlled by Lr34/Yr18 and Sr2 that are linked with morphological markers can be more easily manipulated in the presence of more effective genes. In absence of any associated marker with partial of incomplete resistance, combination of these two distinct types of resistance can be achieved by following a two step breeding strategy. For stripe rust resistance, resistances that were transferred from durable resistant cultivars across widely different genotypes into the formerly popular wheats WL 711 and Kalyansona, offer well adapted materials for imparting durable resistance, in addition to use of the gene Yr18.