a thread the other day got me to thinking about what's been posted up on this issue in the past (not alot from what I can gather)   http://arbtalk.co.uk/forum/tree-health-care/70269-help-damage-limes.html     Definiton; fungal associated die back of the bark and cambium on stems (sometimes on branches) showing as a 'target' where annual callus material is layed down in an attempt to compartmentalize the infected area.   Often focused at a point of a failed branch attachment or stub.