Genetic variability and the inheritance of resistance to cedar‐apple rust in apple

Thirteen apple cultivars and selections were evaluated in a greenhouse for their reaction to cedar‐apple rust inoculation Five of the cultivars selections developed aecial lesions on their leaves whereas the sixth cultivar Jonathan, showed only pycnial lesions. The responses of the remaining seven resistant cultivars selections ranged from a lack of any visible symptoms to the presence of discrete flecks or continuous chlorotic mottles Quantitative parameters were used for statistical analysis; these included the highest number of pycnial lesions on a leaf/plant (N). the largest diameter of pycnial lesions on any leaf/plant (D)), and the calculated infection rating (IR = ND). Significant differences were observed among cultivars selections for variables N. D. and IR. Segregation ratios among progenies derived from 16 controlled crosses indicated that resistance to cedar‐apple rust was conditioned by two major genes, and both were required in dominant form to confer resistance. However, in three crosses the dual‐gene hypothesis was inadequate to explain the reaction of seedlings.