A large bioassay identifies resistance genes that provide broad resistance against Septoria tritici blotch disease in the UK.

Introduction: Septoria tritici blotch (STB) is one of the most damaging fungal diseases of wheat in Europe, largely due to the paucity of effective resistance genes against it in breeding materials. Currently dominant protection methods against this disease, e.g. fungicides and the disease resistance genes already deployed, are losing their effectiveness. Therefore, it is vital that other available disease resistance sources are identified, understood and deployed in a manner that maximises their effectiveness and durability.

Methods: In this study, we assessed wheat genotypes containing nineteen known major STB resistance genes (Stb1 through to ) or combinations thereof against a broad panel of 93 UK isolates. Seedlings were inoculated using a cotton swab and monitored for four weeks. Four infection-related phenotypic traits were visually assessed. These were the days post infection to the development of first symptoms and pycnidia, percentage coverage of the infected leaf area with chlorosis/necrosis and percentage coverage of the infected leaf area with pycnidia.

Results: The different genes were found to vary greatly in the levels of protection they provided, with pycnidia coverage at four weeks differing significantly from susceptible controls for every tested genotype. , , , , , and were identified as contributing broad spectrum disease resistance, and synthetic hexaploid wheat lines were identified as particularly promising sources of broadly effective STB resistances.

Discussion: No single isolate was found to be virulent against all tested resistance genes. Wheat genotypes carrying multiple genes were found to provide higher levels of resistance than expected given their historical levels of use. Furthermore, it was noted that disease resistance controlled by different genes was associated with different levels of chlorosis, with high levels of early chlorosis in some genotypes correlated with high resistance to fungal pycnidia development, potentially suggesting the presence of multiple resistance mechanisms.The knowledge obtained here will aid UK breeders in prioritising Stb genes for future breeding programmes, in which optimal combinations of resistance genes could be pyramided. In addition, this study identified the most interesting Stb genes for cloning and detailed functional analysis.