A single gene controls resistance to septoria nodorum blotch in the Aegilops tauschii accession AUS21712

A potential source of resistance to septoria nodorum blotch had been identified in an accession of the wild wheat, Aegilops tauschii. A cross was made between the resistant Ae. tauschii accession, AUS21712, and a susceptible accession, CPI110889, to study the genetics of resistance. The parental accessions and the F1, F3, and F4 progeny were screened in the glasshouse as seedlings. The resistant parent took significantly longer to develop symptoms, developed significantly fewer lesions, and expressed significantly lower levels of disease than the susceptible parent. The F1 mean response for disease severity indicated resistance was dominant. The genotypic ratios generated from the screening of the F3 and F4 generations were not significantly different from the genotypic ratio expected for a single gene. The efficacy of the resistance and its simple genetic control in the Ae. tauschii accession AUS21712 means that the potential exists to use this Ae. tauschii resistance gene in a bread wheat breeding program.

Detection of Virulence to Lr41 in a South African Pathotype of Puccinia recondita f. sp. tritici

The Lr41 gene for resistance to leaf rust, caused by Puccinia recondita f. sp. tritici, was transferred from Triticum tauschii accession TA2460 to bread wheat (T. aestivum) at the Wheat Genetics Resource Center, Kansas State University (1). Due to the high degree of resistance (primary leaf infection types [ITs] 0; to ;1= [0 to 4 scale]) conferred to South African pathotypes of leaf rust, the gene was considered potentially valuable in local programs directed at wheat germ plasm improvement. During routine glasshouse tests for seedling resistance to the South African leaf rust pathotype UVPrt16, several sporulating pustules (IT 4) were observed among flecks on wheat line KS90WGRC10 (TAM107*3/TA2460). Rust cultures multiplied from these pustules were tested on seedlings of 31 wheat lines with different Lr genes in a growth cabinet at 20°C. Their avirulence/virulence characteristics suggested a single-step mutation for virulence to Lr41. According to North American nomenclature (2), the Lr41-virulent isolate is classified as race FDC-14b,20,23,25,33,41. Low ITs were recorded on additional testers with Lr3bg, 14a, 15, 18, 19, 27+31, 28, and 29, and intermediate ITs on lines with Lr2b, 10, 21, 32, and 36. Infection studies on adult plants showed that KS90WGRC10 and its recurrent parent TAM107 produced similar susceptible reactions (Its 3++C) to the mutant. T. tauschii accession TA2460, however, was moderately susceptible (IT 2+) to this culture. Virulence to Lr41 has not been recorded in field populations of the pathogen in South Africa. The occurrence of virulence to Lr41 emphasized the nondurability of monogenic resistance and thus the importance of conserving genetic diversity for leaf rust resistance. To protect Lr41 from becoming obsolete, it should be used only in combination with other effective resistance gene(s). References: (1) T. S. Cox et al. Crop Sci. 32:506, 1992. (2) D. L. Long and J. A. Kol-mer. Phytopathology 79:525, 1989.