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. Photo courtesy A. L. Shigo.