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Resistance

Influence of Genetic Background on the Expression of Wheat Leaf Rust Resistance Gene Lr22a. Z. A. Pretorius, Grain Crops Research Institute, Bethlehem 9700, South Africa, Present address: Department of Plant Pathology, University of the Orange Free State, Bloemfontein 9300, South Africa; F. H. J. Rijkenberg(2), and R. D. Wilcoxson(3). (2)Department of Microbiology and Plant Pathology, University of Natal, Pietermaritzburg 3200, South Africa; (3)Department of Plant Pathology, University of Minnesota, St. Paul 55108. Phytopathology 80:579-584. Accepted for publication 21 December 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-579.

F₄ families homozygous for Lr22a, derived from crosses between the wheat genotypes RL6044 (Thatcher:Lr22a) and Zaragoza or SST33 (leaf rust-susceptible) were developed. Quantitative assessment of latent period of Puccinia recondita f. sp. tritici and number of uredinia per centimeter of square leaf surface of these families indicated significantly different levels of resistance. Although infection types, resulting from inoculations with different isolates of known virulence, denoted the presence of Lr22a, certain F₄ families exhibited a latent period statistically equal to that of the susceptible control. Other F₄ families showed resistance superior to that of RL6044. Selected F₅ families indicated that levels of adult-plant resistance superior to that of the parent could be maintained in a selection program. Enhanced levels of resistance were apparently not caused by the combination of Lr22a with other known, but ineffective genes for hypersensitive seedling resistance in the susceptible parents, or by the growth stage of plants in families segregating for maturity. It is hypothesized that modifying genes, which cannot be detected qualitatively, interacted with Lr22a to produce different levels of resistance. These modifying genes were not associated entirely with Lr22a since F₄ families without the gene also varied for latent period and the number of uredinia.