Race structure of Pyrenophora tritici-repentis isolates obtained from wheat in South America

Knowledge of genetic variation in a pathogen population contributes to breeding for disease resistance. The fungus Pyrenophora tritici-repentis, cause of tan spot of wheat, is an important foliar pathogen worldwide. Currently, eight races have been identified in the fungal population prevalent on wheat and alternative hosts. Races 1 through 6 have been observed in North America. However, the fungal population from South America has not been characterized as to race. In this study, 48 single-spore isolates of P. tritici-repentis, recovered from wheat, were obtained from Argentina (10), Brazil (23), and Uruguay (15). Isolates were tested by inoculating individually on 2-leaf stage seedlings of the wheat differentials Glenlea, Katepwa, ND495, 6B365, M-3, and Salamouni in the greenhouse. They were grouped into different races based on necrosis and chlorosis induction on the differentials. Isolates from Argentina were grouped into races 1 and 7; from Brazil into races 1 and 8; and, from Uruguay into races 1 and 2. Results indicate that P. tritici-repentis has a diverse population on wheat in South America. More isolates are under investigation to obtain a comprehensive virulence pattern of the pathogen population in South America. Wheat lines should be screened against all known races to achieve durable resistance in a cultivar release program. The P. tritici-repentis race structure on wheat in Europe needs to be determined, as the pathogen has become a concern for wheat breeders and pathologists.