Cherry stem showing gummosis. The picture shows a young branch of a cherry tree, Prunus avium, that is exuding a resinous gum (dark amber fluid). This is common behaviour in Prunus species, especially cherries, and can have several causes. The least serious is simply as a reaction to climatic conditions, especially a long period of drought. However, gummosis (resinosis) is also, as here, a symptom of bacterial canker, caused by Pseudomonas syringae, pathovar morsprunorum. This is a bacterium that gains access to the plant tissues through pruning cuts, leaf scars or mechanical damage. It causes cankers - shallow depressions in the stem due to failure of the underlying tissues to grow. In this picture shallow cankers obscured by gum are visible at the extreme top and bottom of the frame