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Characterization of Adult-Plant Resistance to Leaf Rust of Wheat Conferred by the Gene Lr22a. Z. A. Pretorius, Senior Agricultural Researcher, Small Grain Centre, Bethlehem 9700, South Africa. F.H.J. Rijkenberg, and Roy D. Wilcoxson. Professor, Department of Microbiology and Plant Pathology, University of Natal, Pietermaritzburg 3200, South Africa; and Professor, Department of Plant Pathology, University of Minnesota, St. Paul 55108. Plant Dis. 71:542-545. Accepted for publication 26 January 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0542.

Inoculation of adult-plant resistant wheat line RL6044 with Puccinia recondita f. sp. tritici established that resistance gene Lr22a could be detected by infection types produced on the third and fourth leaves of 30-day-old plants but not on the third leaf of 23-day-old plants. With six isolates of the pathogen from the Cereal Rust Laboratory, the mean latent period of RL6044 was 40 hr greater than that of line E, a susceptible line. Density of uredinia was not reduced, but their mean size on flag leaves of RL6044 was about 70% less than on line E. With four South African isolates on flag leaves, the mean difference in latent period between RL6044 and line E was 134 hr. Urediniospore production 19 days after inoculation was about 89% lower on RL6044 than on the susceptible cultivar Morocco. The absence of an interaction between individual resistance components of Lr22a and different races of the wheat leaf rust pathogen suggest that this gene may be useful for managing epidemics of P. recondita f. sp. tritici.