I have been mulching close to my trees for years. Just be sure it stays away from the bark -- an inch or so is enough. Taper the depth so that it goes from nothing at the soil line of the trunk to your desired depth a minimum of 6-8 inches out from the trunk. That works for me. I haven't noticed any significant difference in terms of either disease or pest issues between trees mulched this way and those not mulched.

What you DON'T want to do is pile up mulch against the trunk -- that creates a wet environment where disease organisms can grow and gives pests, both small and large (voles being "large") a place to hide.

FWIW, if you make sure the area is free of weeds, weed roots and bits that can re-grow, you surely would be safe in using a little Preen to keep weed seeds from germinating.

I put down some sheets of newspaper that go right to the trunk and pull away the mulch.