A hackberry tree, Celtis occidentalis. Notice the clumpiness in the canopy?
A slightly closer view of one fo the clumps. (They were all out of hand’s reach.) This is witches’ broom, a gall-like growth of branches sprouting in multiples. Hackberry is particularly susceptible. In this case, it seems to be caused by a combination of a fungus and a mite. I gather the mite carries the fungus…
It is thought to be:
“attributed to two agents acting together: a powdery mildew fungus (Sphaerotheca phytoptophila) and a minute, wormlike, eriophyid mite (Eriophyes celtis, synonym Aceria snetsingeri) about 200 microns long.”
The mite’s name has recently been changed to Aceria celtis.
Evidently there are strains of hackberries that are immune to it. People think it’s unsightly. People!
This specimen is in Prospect Park. There are two young hackberries outside our window here. You may remember that the city’s street tree survey insisted they were hawthorns. We fixed that. Just the other day I noticed that one of them had some witches’ broom in it. I don’t think the tree had any there last year. I’m not positive, but I don’t feel like they were there.
Backyard and Beyond 
I have a cedar tree in my Ozark acres that has this condition.