Identification of quantitative trait loci for race-nonspecific resistance to tan spot in wheat.

Tan spot, caused by Pyrenophora tritici-repentis (Ptr), is an economically important foliar disease in the major wheat growing areas throughout the world. Multiple races of the pathogen have been characterized based on their ability to cause necrosis and/or chlorosis on differential wheat lines. In this research, we evaluated a population of recombinant inbred lines derived from a cross between the common wheat varieties Grandin and BR34 for reaction to tan spot caused by Ptr races 1-3 and 5. Composite interval mapping revealed QTLs on the short arm of chromosome 1B and the long arm of chromosome 3B that were significantly associated with resistance to all four races. The effects of the two QTLs varied for the different races. The 1B QTL explained from 13% to 29% of the phenotypic variation, whereas the 3B QTL explained from 13% to 41% of the variation. Additional minor QTLs were detected but not associated with resistance to all races. The host-selective toxin Ptr ToxA, which is produced by races 1 and 2, was not a significant factor in the development of disease in this population. The race-nonspecific resistance derived from BR34 may take precedence over the gene-for-gene interaction known to be associated with the wheat-Ptr system.