There are wart-like growths on the undersides of the leaves on my hackberry. What are they?

Question:

There are wart-like growths on the undersides of the leaves on my hackberry. What are they?

Answer:

The wart-like growths on the hackberry leaves are galls.  Galls are abnormal growths of plant tissue induced to form by mites, insects, or other small organisms.  They are quite common on trees.  

The gall found on the hackberry leaves is referred to as the hackberry nipple gall.  The gall is induced to form by a gnat-like psyllid.  The hackberry nipple gall is so common on hackberries that its presence can be used to identify the tree.  

While galls may be unsightly, they do not cause serious harm to healthy trees.  Preventive insecticide treatments are seldom warranted.