Formation of Stromata and Cankers John Pinkerton, Ken Johnson, Jeff Stone The phloem and cambium that is colonized by the fungus develop a necrotic, chocolate brown discoloration in the spring as the tree resumes growth. Elliptical stromata of the pathogen begin to develop within the discolored tissues in April. Fungal cells form a pressure cushion within the primary phloem. The pressure cushion expands crushing the phloem and raising the bark on the surface. This is seen as a series of bumps along the branch. The pressure cushion becomes the stroma and erupts through the bark in late May to early June. The stromata are at first white when they break through the surface and turn black later in the year. The greatest length of the stroma is aligned with the long axis of the stem. Stromata usually are arranged in single or double rows along the stem. Death of the cambium in the area of the canker results in a sunken appearance as the surrounding cambium continues to grow. New cambium forms at the edge of the pressure cushion. A callus is laid down, walling off the infected region from the healthy tissue.   Immature stromata that have erupted throught the bark in early summer. Photo by Jay W. Pscheidt, 1990				Life Cycle Tour Previous / Next / Animated     The very first symptom can be seen (or felt with your fingers) in the late spring or early summer as rows of bumps along a branch. The stromata have not quite broken through the bark yet. Photo by Jay W. Pscheidt, 1996
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