Cytospora Canker

Cytospora canker manifests as dead twigs and branches in spring, with foliage above the infection turning brown while remaining attached to the stem (creating "flags"). The canker itself appears as a sunken, elongated lesion in the bark, often with visible pinhead-sized fruiting bodies; in humid weather, you may spot amber or orange spore tendrils extruding from the bark. The fungus is vigorous in exploiting wounds from winter injury, sunscald, frost cracks, or pruning, and once it establishes in the cambium, it slowly girdles the stem. This disease affects pome fruits, stone fruits, and ornamental trees including oak and aspen.

Cytospora canker becomes a serious problem when trees are stressed and wounded, particularly in spring and early summer as the fungus resumes activity. Minimize damage by avoiding pruning wounds during the growing season, training trees properly to reduce branch breakage, and protecting against rodent damage and frost crack. Do not over-apply nitrogen or irrigate late in the season, as vigorous growth followed by cold snaps creates ideal conditions for canker formation. If you spot infected branches, prune them off at least a foot below any visible discoloration, sterilizing tools between cuts, and avoid establishing new orchards near badly diseased ones where inoculum loads remain high.