White cotton underside of leaves
First of all, what zone is southern Maine?
I have a new Serviceberry tree that has a few handful of leaves that are yellow and falling off. On some of those leaves and others, on the underside of the leave is a white cotton-like substance that you can wipe off with your finger. The tree gets morning sun and here in Maine it has been rainy, humid, cold then hot, etc. Any ideas as to what this could be and do I need to do anything about it?
Some thoughts are downy mildew, planthopper, scale.