VIEW ARTICLE

Resistance

Nature and Inheritance of Resistance to Gymnosporangium juniperi-virginianae in Apple Cultivars. Herb S. Aldwinckle, Associate Professor, Department of Plant Pathology, New York State Agricultural Experiment Station, Geneva, NY 14456; R. C. Lamb(2), and H. L. Gustafson(3). (2)Associate Professor, Department of Pomology and Viticulture, New York State Agricultural Experiment Station, Geneva, NY 14456; (3)Research Technician, Department of Plant Pathology, New York State Agricultural Experiment Station, Geneva, NY 14456. Phytopathology 67:259-266. Accepted for publication 9 September 1976. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-259.

Fifty-eight apple (Malus pumila) cultivars and numbered selections were inoculated artificially with a population of Gymnosporangium juniperi-virginianae (the incitant of cedar apple rust) of wide-host-range. The most advanced reaction (chlorotic mottle, flecks, pycnia, or aecia) was recorded for each cultivar, and maximum and mean infection ratings for each cultivar were determined. Analysis of 15 progenies from crosses of selections derived from M. pumila, M. floribunda, M. prunifolia, and M. toringo supports the hypothesis that resistance (absence of pycnia) is controlled by two genes. The genes, designated Gy-a and Gy-b are phenotypically indistinguishable. Both genes are required in dominant form for suppression of pycnia. Leaf damage, expressed as infection ratings, is not predicted reliably by the dual-gene hypothesis and is probably controlled by Gy-a, Gy-b, and several modifying genes. A virulence formula similar to those used for wheat rusts is proposed to describe the pathogenicity of a given population of G. juniperi-virginianae. A list of sequential numbers for current differential cultivars is presented. Virulence formulae already observed have been assigned formula numbers.