Figs. 3.10-3.13. Fig. 3.10. Penetration of the bark of yellow birch by the canker rot pathogen Inonotus obliquus. The pathogen produces a wedge of fungus material into the bark. The larger white arrows show the boundary of the wedge that penetrated the bark three years before the tree was dissected. The smaller white arrows show an earlier boundary. The double black arrows in the bark show the tip of the wedge. The white pointer indicates the position where the wedge meets the living cambium. The black arrows in the xylem indicate the barrier zone formed in response to infection. Fig. 3.11. Transverse section of a Strumella canker on red oak showing the series of annual callus rings typical of most perennial target-shaped cankers. The smaller, black arrows drawn on the wood show the xylem boundary formed during the seasons before this section was cut. The white arrows show the pathogen during the current season, circumventing the bark boundaries and invading previously healthy bark. Fig. 3.12. Section of aspen with Hypoxylon canker showing a xylem boundary in the bark (white arrow). Fig. 3.13. Section of aspen with Hypoxylon canker taken from the vertical tip of the canker and showing the old infected bark beneath the back of the white arrow. (Photographs courtesy of A.L. Shigo).