Possibly a canker?
a) Nectria canker - small areas of bark, often close to a bud or a wound, darken and sink inwards; the bark cracks forming loose flaky concentric rings. In extreme cases the entire shoot becomes ringed & dies back. Cause - Nectria galligena spores infecting wounds caused by pruning, leaf fall, irregular growth, frost, wooly aphids. Treatment - prune/remove affected area, treat with canker paint containing cresylic acid. Spray tree with appropriate fungicide.
b) Bacterial canker - flat or sunken, elongated cankers develop on the stems; droplets of golden coloured gum ooze from the affected area. Cause - bacteria Pseudomonas syringae / p. mors-prunarus. Treatment - remove cankered branches, treat wounds with a wound paint. Spray the foliage with Bordeaux mixture/other copper fungicide in late summer/early autumn.
