We have maple trees. Huge old maples that provide a lot of shade (and maple syrup! – more on that coming in the colder months I promise). This year however I noticed some odd little spikes on the leaves.
These are called maple spindle gall mites (Vasates aceriscrumema). The spikes are growths that the leaves produce in response to a Gall Mite infestation. It may cause some leaves to drop off but generally no harm is caused to the tree. It will not affect the sap production. There are treatments available, however it’s not necessary unless the infestation is so severe the tree is covered completely.