Question
i recently purchased a house in brooklyn, ny with what i
think is an oak tree in the back yard. its a mature tree -
taller than the 3 story house. i've noticed that many of the
leaves that have fallen during the summer have what look
like pimples all over them.

any idea what this is? should i be concerned for the tree's
overall health?

thanks, Mischa

Answer
This sounds like gall mites on the leaves. These tiny insects make small galls on the leaf when they lay their eggs. These are not a large concern to the overall health of the tree. I would rake up the falling leaves and destroy them this should help cut the popluation next year. This is just a leaf problem that attacks late in the summer and the only effect will be loss of leaves earier than notmal. No control is necessary on a large tree.