Race Diversity of Pyrenophora tritici-repentis in South Dakota and Response of Predominant Wheat Cultivars to Tan Spot

Abstract

The fungus Pyrenophora tritici-repentis (Ptr) causing tan spot (TS) is an important pathogen of wheat in the US Northern-Great-Plains. Knowledge of physiological variation in the pathogen population is essential in the development of durable TS resistant cultivars. Eight Ptr races have been identified based on three host selective toxins (Ptr ToxA/Ptr ToxB/Ptr ToxC), which are associated with necrosis and chlorosis symptoms. The information about Ptr race structure and reaction of wheat cultivars grown in SD to tan spot is scarcely available. In this study, 569 isolates of Ptr collected from wheat were genotyped for Ptr ToxA and Ptr ToxB genes and a subset of 134 isolates were evaluated for their race identity on a wheat differential set. Ptr ToxA and Ptr ToxB genes were amplified in 89.6% and 0.4% isolates, respectively. The remaining 57 (10%) isolates lacked both toxins genes. The characterization of 134 isolates exhibited diverse race structure with 74.6%, 18.7%, 1.49% , and <1% isolates categorized as race 1, 4, 5, and 2, respectively. Another six (4.5%) isolates behaved like race 2 but lacked Ptr ToxA gene, hence could not fit under the currently known eight races. Our results determine the diversity of Ptr population that exists in SD and establish the presence of race 5 in SD for the first time. Since races 1 and 5 are most prevalent in the region, we screened 45 most predominant wheat cultivars against these races and Ptr ToxA. We observed eleven cultivars resistant or moderately resistant to both races, however, seven spring wheat cultivars showed susceptibility to both races 1 and 5. Continued cultivation of wheat cultivars susceptible to both races could play a role in the establishment and development of new races. Continuous germplasm enhancement and periodically monitoring Ptr population can help in better TS management. Keywords: