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- Wheat is the UK's major food crop. A major constraint on wheat production is the disease yellow rust (YR), caused by the fungus Puccinia striiformis f.sp. tritici (Pst), with yield losses up to 50% in untreated crops. The two major control measures for this disease, used in combination, are use of resistant varieties and application of fungicides. Fungicide application is effective but expensive, limited by weather conditions and increasingly restricted in use due to environmental concerns. Host (i.e. wheat) resistance can also be very effective, with several known resistance genes conferring immunity to known races of Pst. However, populations of the pathogen regularly change and resistance genes suddenly become ineffective when the pathogen mutates. In Europe, there was a recent rapid incursion of an exotic Pst population, which is much more diverse than the established population it has displaced. As a result, there have been dramatic and ongoing changes in the patterns of YR resistance in commercial wheat varieties. In 2016 alone, seven varieties on the UK wheat Recommended List had their resistance ratings substantially reduced, with significant cost impact on growers. Breeding improved wheat varieties with effective, long-lasting YR resistance to withstand current and future incursions is now a top priority for northern European (NE) wheat breeders. In Yellowhammer, we will employ a strategy based on detecting and utilizing multiple race non-specific adult plant resistance (APR) genes, for long-term genetic control of the disease. These genes usually confer partial resistance, are characterized by reduced and slower pathogen growth, and can be 'stacked' with each other or with 'major" genes in the same plant to provide effective long-lasting resistance. We previously identified several APR genes - but finding the most effective combinations is challenging as different genes interact with each other in complex ways. To address this challenge, we are collaborating with seven NE breeding companies and the UK's Agriculture and Horticulture Development Board to develop experimental wheat populations based on elite European varieties, but which differ in the combinations of YR APR genes they carry. We will use these to: 1. Identify the most effective combinations of APR genes, and the times of the season they become effective, in field tests at twelve sites in NE over four years. We will investigate what is the most effective combination of strong and weak APR genes to achieve YR resistance in wheat. We will also determine the effectiveness of APR genes in hybrid wheat and any side-effects APR genes have on grain yield. 2. Determine, using 'microphenotyping', the timing and location of action in the plant of different APR genes involved in the pathogen-host interaction, helping us select functionally complementary APR genes to combine. 3. Identify which wheat genes and genetic pathways are switched on or off in response to the pathogen in the presence of different APR gene combinations in order to understand how to best assemble APR gene combinations with complementary molecular genetic mechanisms of resistance. We will also conduct field pathology trials of newly available populations and varieties to identify new APR genes. The results will allow us to determine the best combinations of resistance alleles to stack, and provide new genetic markers to aid the process. Results will be validated in active commercial breeding material and will immediately be translatable into the breeding programs of our commercial partners, enabling them to breed more durably resistant wheat varieties equipped to resist the current Pst population and potential future incursions. This will improve UK arable production and food security, and reduce environmental harm, as farmers benefit from having access to more consistently performing, longer-lasting varieties with reduced fungicide requirements.