Hi Karen,

From what I can see in the photo, those look like plant galls, caused by wasps.  They are distortions of tissue that grow around the developing insects; usually they are initiated during leaf expansion.  They are usually considered to be a cosmetic problem, not something that will damage the tree long-term.  In nearly all cases, prevention of gall
formation is exceedingly difficult and is not considered practical.  Systemic insecticides may offer some measure of control, but the damage caused by the gall is usually not enough to warrant the expense and effort.

You can prune off any particularly affected leaves or branches too, to improve appearance and, if you get them before they've emerged, remove next year's crop of wasps from your property.

I hope this helps!

Cordially,

John