Comparative virulence of chlorosis-inducing races ofPyrenophora tritici-repentis

The fungal pathogen Pyrenophora tritici-repentis (Died.) Drechs. causes tan spot, an important foliar disease of wheat worldwide. To date, five races of this pathogen have been described. We report the existence of a sixth race and compare the chlorosis symptoms produced by various isolates of P. tritici-repentis representing different races. Isolate Alg-H2, collected in eastern Algeria, has a virulence pattern that combines those of races 3 and 5. Thus, we propose that this isolate be classified in a new race, designated race 6. Isolate 92-171R5, collected on the Manitoba—Saskatchewan border, possesses the same virulence pattern as race 5 isolates but is much less aggressive based on the size of the lesions and the extent of the chlorosis it causes. Ptr ToxB was purified from Alg-H2 culture filtrate, following essentially the same protocol as reported earlier for its purification from race 5 isolate Alg 3-24. The presence of a toxic principle, with the same specificity as Ptr ToxB, was also demonstrated in 92-171R5 culture filtrate.