Verticillium Wilt

• Verticillium albo-atrum and Verticillium dahliae are the two types of fungi that cause verticillium wilt in plants like the black locust tree. These fungi are soil-borne pathogens that invade a plant through its roots and begin affecting the tree from the inside out. Early symptoms of infection include wilting, discoloration of the leaves and premature leaf drop. Treat verticillium wilt by pruning the damaged branches and apply an ammonium sulfate fertilizer to the soil, according to the Virginia Cooperative Extension.
  
  

Locust Borer

• Locust borers are insects that only attack black locust trees, according to the Washington State University Cooperative Extension and the Trees for You website. The locust borer lays its eggs in crevices in the bark of the tree; when the larvae hatch, they borrow further into the tree and begin feeding on the wood. Severe infestations cause dieback of the branches. Control locust borer populations with pesticides in the fall to prevent further infestations from occurring. Prune damaged branches from the tree.
  
  

Cankers

• Botryosphaeria dothidea is a type of canker-causing pathogen that attacks over 100 tree species, including the black locust, according to the University of Illinois Extension website. The disease primarily attacks trees that are already stressed or weakened, as from problems with the locust borer or verticillium wilt. Cankers form on young or weak branches and lead to dieback. Pruning damaged branches and keeping the growing environment healthy are key to preventing and solving problems with canker.
  
  

Wood Rot Fungi

• Many fungal species lead to wood rot, according to the University of Illinois Extension. These fungi produce no external symptoms until the inside of the tree is already dead; at this point, branch dieback is more common and shelf or conk fungi appear on the trunk and branches of the tree. The fungi only appear on wood that is already dead, so the appearance of these fungal growths indicates a need to cut down and replace the tree, which can otherwise become a safety hazard.