Selection of Puccinia recondita f.sp. tritici virulence phenotypes in three multilines of Thatcher wheat lines near isogenic for leaf rust resistance genes

A heterogeneous population of Puccinia recondita f.sp. tritici developed from randomly mated pycnial infections on Thalictrum speciosissimum was selected for 12 generations on three multilines composed of Thatcher wheat lines near-isogenic for leaf rust resistance genes. Multiline 1 was composed of 20% each of lines near-isogenic for resistance genes Lr2a, Lr3ka, Lr11, Lr17, and Lr24. Multiline 2 was composed of equal proportions each of Thatcher and the five near-isogenic lines in multiline 1. Multiline 3 was composed of 50% Thatcher and 10% each of the five near-isogenic lines. Diversity of virulence phenotypes as measured by the Shannon index declined most in the population selected on multiline 1. Phenotypes of P. r. tritici with virulence to three of the five resistance genes in the multilines were the predominant isolates in all three populations after 12 generations of selection. Frequency of phenotypes with virulence to resistance genes Lr2a and Lr11 significantly increased in all three populations. Virulence to Lr11 significantly increased in the multiline 3 population, and virulence to Lr24 increased in populations from multilines 2 and 3. Frequency of phenotypes with virulence to Lr3ka did not significantly change in any of the populations. Virulence phenotypes with an intermediate number of virulences appeared to have fitness advantage relative to other isolates on the three host multilines. These results indicate that use of wheat multilines may not necessarily stabilize selection for virulence complexity in P. r. tritici populations. Key words: mixtures, Triticum aestivum, wheat leaf rust.