Seedling resistance to tan spot and Stagonospora nodorum blotch in synthetic hexaploid wheats

Tan spot and Stagonospora nodorum blotch (SNB), caused by fungi Pyrenophora tritici-repentis (Died.) Drechs. fanamorph: Drechslera tritici-repentis (Died.) Shoem.] and Phaeosphaeria nodorum (E. Mwler) Hedjaroude [anamorph: Stagonospora nodorum (Berk.) Castellani & Germano], respectively, are two important foliar diseases of wheat (Triticum aestivum L.). The objective of this study was to evaluate the two sets of elite synthetic hexaploid wheat (SHW, 2n = 6x = 42, AABBDD) lines (Elite 1 and Elite 2) developed at the International Maize and Wheat Improvement Center (CIMMYT) for their seedling resistance to P. tritiei-repentis and P. nodorum. In this study, 120 elite CIMMYT SHW lines and their durum wheat IT. turgidum subsp. durum (Desf.) Husn.] parents were inoculated with P. tritici-repentis race 1 and a standard field isolate (Sn2000) of P. nodorum, respectively, in two separate three-replication experiments. The seedling reactions to P. tritici-repentis and P. nodorum were evaluated 7 and 10 d postinoculation, respectively. The plant leaves were also infiltrated with the host-selective toxin (HST) Ptr ToxA at the two-leaf stage and sensitivity was evaluated 3 to 4 d postinfiltration. As expected, most SHW lines were the same as their durum parents in their sensitivity to Ptr ToxA because the sensitivity locus Tsn1 is located on chromosome 5B. However, a few of the synthetics were different from their durum parents, suggesting that heterozygosity and heterogeneity might exist in some of the SHW lines and durum parents. The toxin sensitivity significantly increased susceptibility of the synthetics to tan spot but had no significant effects on durum parents. The data showed that 56 (46.7%) and 36 (30.0%) SHW lines were resistant to tan spot and SNB, respectively, whereas resistance was almost absent in the durum parents. These results suggest that the elite CIMMYT synthetics are an excellent source of resistance to tan spot and SNB and should be useful in developing new resistant cultivars and adapted germplasm in bread wheat.