ADDITIONAL SOURCES OF RESISTANCE TO TAN SPOT OF WHEAT

Tan spot, a foliar disease of wheat (Triticum aestivum L. em Thell) an d durum (T. turgidum var. durum L.) caused by Pyrenophora triticirepen tis (Died.) Drechs., reduces yield in susceptible cultivars by 3 to 50 %. Additional sources for resistance to tan spot were sought in germpl asm consisting of a collection of spring wheats previously screened fo r reaction to spot blotch [Cochliobolus sativus (Ito & Kurib.) Drech. ex Dastur] in Brazil, wheat cultivars recommended for cultivation in t he state of Parana, Brazil, and a collection of synthetic hexaploid wh eats. Plants from each of the three groups were inoculated at the seed ling stage with a composite of four fungal isolates and at the adult s tage 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 reaction s did not always agree with seedling evaluations and some of the varia tion could be explained by sensitivity or a mixed reaction to necrosis toxin. Assay with purified necrosis toxin was a reliable way to detec t a major factor involved in resistance. This work identifies diverse germplasm available for incorporation of tan spot resistance into cult ivated wheats.