Tan spot, caused by Pyrenophora tritici-repentis (Ptr), is an important disease of wheat worldwide and primarily managed by fungicides. Thirty-five baseline isolates and 136 isolates collected from pyraclostrobin-treated fields were used to determine the effective fungicide concentration that inhibited conidia germination by 50%. The results indicated that the field exposed isolates have not shifted to reduced sensitivity to pyraclostrobin. The second objective was to develop a diagnostic method for the four known mutations responsible for the shift in the sensitivity to QoI in the Ptr. Twelve specific primers were designed to clone a portion of the mutant cyt b in order to serve as controls. The restriction enzyme digested PCR product of known insensitive isolates of Ptr and reduced sensitive isolates of Ptr and P. teres had DNA bands for nucleotide substitution at G143A and F129L, respectively which was identical to cyt b mutant clones. This method is rapid and specific to monitor QoI insensitive and reduced sensitive isolates of Ptr.

A total of 535 spring wheat accessions from around the world were inoculated with the ToxA lacking Ptr isolate. Association mapping (AM) was used to identify quantitative trait loci (QTL) involved in resistance to tan spot. AM using two QK mixed models identified 11 QTL responsible for resistance to tan spot and these QTL together accounted for 14.4% of the phenotypic variation. Mapping populations can be developed from resistant wheat accessions and used to find the closest markers linked to putative genes which will be useful in wheat breeding.

Another aim was to partially purify the necrosis causing toxin produced by AR CrosB10. The crude culture treated with heat and proteinase showed that toxin can remain active at least at 100°C and novel toxin appears not to be a protein, respectively. Partial purification procedure using C-18 column collected the toxin activity in the mobile phase washed by the 70% methanol. The new race can identify using wheat differential Erik, which is sensitive to the crude culture induced by AR CrosB10 but insensitive to other known toxins. This new toxin is proposed as a Ptr ToxD.