Infected trees in older orchards gradually lose productivity and slowly decline. The fungus attacks woody parts of stone fruit trees through bark injuries, pruning cuts, dead shoots and buds. Visible first is the exudation of gum at the point of infection. The canker forms from a small necrotic center that slowly enlarges with the collapse of the inner bark tissue. Cankers enlarge more along the length than the width of the branch.
Older cankers are therefore oval to elongated in outline.
The outer bark of new cankers usually remains intact, except at points of gumming. In older cankers the bark in the center becomes torn. The gum turns black from alternate wetting and drying and from the presence of saprophytic fungi. Older cankers are surrounded by a roll of callus tissue. Each year the canker enlarges by repeated invasion of healthy tissue.