Perennial Canker of Apple: Seasonal Host Susceptibility, Spore Production, and Perennation of Cryptosporiopsis perennans in Infected Fruit in Eastern Washington. G. G. Grove, Associate Plant Pathologist, Washington State University Tree Fruit Research and Extension Center, Wenatchee 98801. F. M. Dugan, and R. J. Boal. Graduate Research Assistant, and Agricultural Research Technologist, Washington State University Tree Fruit Research and Extension Center, Wenatchee 98801. Plant Dis. 76:1109-1114. Accepted for publication 25 June 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-1109.

Studies on the biology of Cryptosporiopsis perennans, causal agent of perennial canker and bull’s-eye rot of apple, were conducted in orchards in eastern Washington during 1987–1990. Conidia were present on apple cankers throughout the year, with numbers peaking during November-February. The incubation and latent periods were 2–3 and 5–6 mo, respectively, on cankers resulting from artificial inoculation. In contrast to cankers resulting from natural inoculation, sporulation on these cankers was highest during the first year of canker growth and then declined. Trees were most susceptible to branch infection during the period from October to March. There were significant (P < 0.01) negative correlations between the number of inoculations that were successful and mean temperature during the month of inoculation in all years of the study. The period of maximum susceptibility coincided with periods of peak conidia production. Using a semiselective medium, C. perennans was isolated from infected apples overwintered on the orchard floor. All isolates were pathogenic to apple fruit in subsequent inoculations. The pathogen was not retrieved from washings of woolly apple aphids removed from apple cankers. Results indicate that perennation of the pathogen occurs on cankers and in fallen fruit, and that winter pruning (coinciding with peak sporulation with host susceptibility) is performed during the period most conducive to the development of new wood infections.