Germplasm Enhancement for Resistance to Pyrenophora tritici-repentis in Wheat

Yellow spot (syn. tan spot), caused by Pyrenophora tritici-repentis, is an important foliar disease of wheat in Australia that causes losses exceeding 50% when conditions are favourable for disease development. Although good progress has been made internationally to understand yellow spot resistance, relatively few resistance genes have been identified and mapped in Australian germplasm and only one (tsn1 on chromosome 5BL) is in general and known use in Australian breeding programs. Although tsn1 is an important yellow spot resistance gene, it doesn’t explain the full spectrum of resistance and there is a significant opportunity to enhance expression of yellow spot resistance through identification of resistance factors other than tsn1. Six doubled haploid (DH) mapping populations (five of which were fixed for tsn1) were screened for yellow spot resistance at the seedling/tillering and adult plant stages at the Department of Agriculture and Food, Western Australia (DAFWA) and the Department of Environment and Primary Industries Victoria (DPIVic) from 2009 to 2012. Four of the above populations were screened at the Department of Agriculture, Fisheries and Forestry Queensland (DAFFQ). Frequency distribution of individuals within each population for various levels of yellow spot resistance was continuous indicating that resistance is conditioned by several genes with partial effects. A few lines within each population consistently showed high levels of resistance probably resulting from a combination of several genes with additive effects. Nine new loci for yellow spot resistance were mapped by the Australian Wheat and Barley Molecular Marker Program (AWBMMP) at the University of Adelaide on chromosomes 1AS, 2AS, 5AS, 5AL, 2B, 4B, 6BS, 7BL and 2D in five of the six populations phenotyped so far. High LOD scores of 9 to 11 have been obtained for some of the QTL with a percentage disease reduction of 24% to 42%. Efforts are now focused on identifying additional yellow spot resistance genes using newly developed populations and rapid phenotyping methods and developing a series of fixed lines, each carrying yellow spot resistance genes from various sources, in elite Australian backgrounds. These materials will provide proof-of-concept for achieving better resistance by pyramiding resistance genes, and they should be directly useful as parents for wheat breeding.