Q. Do you have idea why some many of the leaves on this Locust tree are turning yellow? A. The most frequent problem with locust trees, Idaho or black locust, is borers in the trunks. This will cause exactly what you’re talking about, yellowing of leaves and leaf drop, followed by branch dieback.             Borers entered the trees usually where it is sunburned and that is on the west and south facing sides of the trunk or limbs. Horizontal limbs may be damaged on the upper surfaces as well.             Check the bark on the trunk or limbs see if it is loose. It may easily pull away from the trunk particularly on the South and West sides. Large limbs may be damaged by sunburn. Sunburn on the trunk of a locust tree             Remove bark away from damage to areas and clean the damage down to fresh wood. You don’t need to paint it but if the customer wants it painted, paint it. Use a liquid insecticide soil drench to help protect the tree and give it a chance to recover.