Septoria nodorum blotch (SNB) and yellow/tan spot (YS) are among the most economically important diseases of bread and durum wheat which are caused by the Dothideomycete fungal pathogen Parastagonospora nodorum (P. nodorum) and Pyrenophora tritici-repentis (Ptr). No resistance mechanism has been reported for the diseases. In this study, a broad SNB and YS resistance wheat line (56:ZWB11) was identified from CYMMIT Australian ICARDA Germplasm Evaluation (CAIGE) collection which was used for developing a double haploid population.

To characterise the resistance, Quantitative Trait Loci (QTL) mapping of the double haploid population was carried out for x SNB and YS related traits including disease, culture filtrate and effector assays; at different plant stages with different pathotypes. Results obtained have revealed the resistance observed in 56:ZWB11 to SNB and YS was complex.

The disease severity was a mixture of resistance and susceptibility conferred by different mechanisms. Physical barrier to both pathogens and susceptibility avoidance due to SnTox267 insensitivity play the major roles in the observed resistance phenotype. QTL profiles were distinct between P. nodorum and Ptr isolates. SnTox267 was found to interact with five wheat loci of which 2DL contributed the largest effect on disease incidence.

In addition, the study identified four other QTL (2A, 2B, 5B and 7B) which are common to multiple isolates and conditions; whereas six and four others only associated with culture filtrates or whole plant infection assays, respectively. Knowledge gained from this study will help to understand the resistance phenomenon and determine which QTL will be focused in the future for SNB and YS marker development. Implication of the findings for future research activities and molecular SNB resistance breeding will be discussed.