Question
We noticed brown fuzzy balls on the leaves, mostly following the midline vein of the leaf.

Answer
Jodie:
These fuzzy balls are probably a type of insect (usually a very very small wasp)that makes a gall on the leaf.  There are lots of different types of "gall makers".  They do not usually have a significant impact on the overall health of the tree, especially if it is established in the landscape and is being taken care of otherwise.  Control usuually does not warrent a chemical.  Just be sure to rake up and discard fallen leaves during the winter and just before spring gets here since these "galled leaves" may be a source for the incest to reoccur in the spring.

Steve