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Erosion of Resistance to Common Leaf Rust in Exotic-Derived Maize During Selection for Other Traits. David W. Davis, Professor, Department of Horticultural Science, University of Minnesota, St. Paul 55108; Cheryl A. Engelkes, and James V. Groth. Graduate research assistant and professor, respectively, Department of Plant Pathology, University of Minnesota, St. Paul 55108. Phytopathology 80:339-342. Accepted for publication 26 October 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-339.

A tropical maize ? Midwest-adapted sweet corn (su) composite (1R) was examined for changes in resistance to common leaf rust (Puccinia sorghi). Eighty-five randomly chosen S1 families from each of three cycles (0, 5, and 10 yr) in the improvement of 1R were evaluated for susceptibility. To encourage recombination for all traits, four generations of random mating and efforts to reduce natural selection were undertaken in the establishment of 1R before cycle 0. Beginning with cycle 1, phenotypic recurrent selection with recombination was conducted each year for various horticultural traits and for disease and insect resistance. No intentional selection was practiced for rust resistance. During the 10 cycles of selection, rust was found on the population each year but always at a low level. If resistance was selectively neutral in the absence of disease, recurrent selection would not have changed the level of resistance. However, mean rust resistance decreased from cycle 0 to cycle 10. This suggests that postponement of selection for rust resistance resulted in the loss of useful genes.

Additional keywords: disease resistance, plant breeding, Zea mays.