Borers

• Several borers, including the lilac borer and the flatheaded apple tree borer can invade crabapple trees. The lilac borer primarily bothers Sargent crab apple trees. The pests invade smaller branches first, causing the trees to become stressed. Small holes in the stems of your crab apple tree's branches indicate borers. Eventually, the tree will die.
  
  

Cankers

• Cankers consist of dead tissue caused by a fungus. The cankers form a ring around some of the branches of the tree's branches. The branches may also contain some dark areas, another sign of cankers. The cankers kill the tree completely over time.
  
  

Root Rot

• Root rot causes decay to form around the base of the tree. If you dig down near the bottom of your crab apple tree and see the decay, your tree will likely die. If the decay is minimal, the tree may survive, but most likely, you need to remove and replace it.