Question
I three large maples in my yard.  One has been slowly loosing it's leaves earlier and earlier each year. This year it had barely  any leaves and lost them within a month.  I then noticed white spots all over the trunk of the tree.  I started looking around and noticed my other two trees have less leaves than usual.  I am very concerned they are dying from a disease.  We live in ottawa ontario canada and the trees are about 30-35 years old.

Answer
Hi Christina,

What you describe sounds like lichens, which are symbiotic relationships between fungi and algae.  They are perfectly harmless and do no damage to plants they grow on.  They will be gray-green in color and will be closely appressed to the trunk.  Look around on other trees and you will probably find them there as well.  No action is required on your part.  They are benign organisms just hanging out on the trunk of your tree.  Let me know if this does not seem to be the case.    

The leaf loss I believe is from a fungus, check out garden.lovetoknow.com/wiki/Maple_Tree_Diseases   and see if this makes sense to you.  Fungal problems can be cured with Atomic Grow which is totally organic.  You can make a drench for the roots to absorb and spray the leaves.  It is simple and very effective.  kathy