Identification of valuable sources of resistance to Zymoseptoria tritici in the Tunisian durum wheat landraces

Tunisia is one of the main producers of durum wheat, the most consumed cereal in Tunisia and represents a trademark of several local dishes such as couscous and bulgur. Nonetheless and despite its leading stand in the local consumption and commercial share, a scarcity in Tunisian durum wheat production has long been a major problem obstructing the fulfilment of the increasing local demand. Septoria tritici blotch (STB) caused by Zymoseptoria tritici (Z. tritici) is one of the most common and detrimental foliar disease on durum wheat in Tunisia causing considerable yield losses. To-date and despite the damaging effects of STB on durum wheat, limited sources of resistance to Z. tritici have been identified. In this present study, we assessed necrosis and pycnidia development of a collection of 304 Tunisian durum wheat landraces representative of 11 landrace populations and artificially inoculated with Z. tritici at the seedling stage under controlled conditions and at the adult plant stage under field conditions. Based on necrosis and pycnidia scores, a hierarchical classification analysis clustered the durum wheat landraces into three phenotypic classes of response to Z. tritici, namely resistant, intermediate and susceptible genotypes. While resistant plants represented the most frequent class at the seedling stage, susceptible phenotypes were more frequent at the adult plant stage. Nevertheless, a heat map correlation study showed that resistant landraces with low-percentages of necrosis and pycnidia at seedling and adult plant stages represented the dominant group within the tested collection. Moreover, the assessed frequencies per landrace populations of the resistant, the intermediate and the susceptible accessions to Z. tritici infection at the adult plant stage for pycnidia development has shown that the landrace populations Badri, Jneh Khotifa (JK), Mekki and Taganrog were the highest resistant accessions, in contrast to the landrace populations Biskri, Mahmoudi, Azizi and Biada where susceptible genotypes were predominant. Mahmoudi and Azizi represented the most variable landrace populations encompassing the three phenotypic classes. Our results highlight that Tunisian durum wheat landraces may harbour variable and stable sources of resistance to Z. tritici at seedling and adult plant stages, which would represent a good basis for further investigation and deployment in breeding programs to enhance STB resistance.