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Progressive Stages of Discoloration and Decay Associated with the Canker-Rot Fungus, Inonotus obliquus, in Birch. Robert A. Blanchette, Assistant professor, Department of Plant Pathology, University of Minnesota, St. Paul 55108; Phytopathology 72:1272-1277. Accepted for publication 5 March 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1272.

The amount of discoloration and decay by the canker-rot fungus Inonotus obliquus (= Poria obliqua) in naturally infected Betula papyrifera was quantified throughout the columns of defect, and progressive stages of degradation were examined by scanning electron microscopy and histological techniques. Massive amounts of thick-walled mycelia penetrated the bark and phloem to incite cankers. The fungus was observed as a pioneer microorganism in the advancing front of the discolored column, colonizing and destroying vessel and parenchyma cell occlusions. Decay was always greatest immediately above and below the sterile conk. Multiple zones of discolored and decayed wood characteristic of I. obliquus decay apparently resulted from repeated attacts by the canker-rot fungus in the phloem and from host response to cambial death and infection. Although compartmentalization occurred, it did not successfully confine the fungus to wood formed prior to wounding, I. obliquus apparently evades and breaks down chemical and morphological barriers produced by trees in response to wounding and infection.