Additional sources of resistance to tan spot of wheat

Tan spot, a foliar disease of wheat (Triticum aestivum L. em Thell) and durum (T. turgidum var. durum L.) caused by Pyrenophora triticirepentis (Died.) Drechs., reduces yield in susceptible cultivars by 3 to 50%. Additional sources for resistance to tan spot were sought in germplasm consisting of a collection of spring wheats previously screened for reaction to spot blotch [Cochliobolus sativus (Ito and Kurib.) Ditch. ex Dastur] in Brazil, wheat cultivars recommended for cultivation in the state of Parana, Brazil, and a collection of synthetic hexaploid wheats. Plants from each of the three groups were inoculated at the seedling stage with a composite of four fungal isolates and at the adult stage with one of the four isolates. A subset of germplasm was treated at the seedling stage with necrosis toxin, crude extract, and conidia produced by P. tritici-repentis isolate 86-124. Adult disease reactions did not always agree with seedling evaluations and some of the variation could be explained by sensitivity or a mixed reaction to necrosis toxin. Assay with purified necrosis toxin was a reliable way to detect a major factor involved in resistance. This work identifies diverse germplasm available for incorporation of tan spot resistance into cultivated wheats.