What is this raised, brown growth on my maple tree?

Question

My tree seems to be infected with something, and every week I notice more leaves have this raised, brown growth. What is it, and how do I treat it?

Answer 1

It's called spindle galls, it is caused by mites. Tiny mites live in these galls, and lay eggs there. It is usually not something to worry about, although the sight of it might not be very nice. Here you can find some more information.