target canker caused by the fungus, Nectria galligena (photo at left). The fungus doesn't attack healthy trees but requires a pruning wound or broken limb to enter the tree and attack the callus tissue that grows to seal over a wound.
We usually don't make massive pruning cuts on pecan trees like the one in the photo, but following the 2007 ice storm, we removed limbs torn off by the weight of the ice. This wound became infected by the Nectria fungus. Each year, the tree forms new callus in an effort to seal over the wound but every year the slow growing fungus kills the callus. The result are concentric rings of dead callus tissue.
A single target canker on a tree seems to have little effect on nut production. However, trees with multiple cankers become obviously weakened and should be thinned from the grove.