One of my Oak Leaf Hydrangeas suddenly began wilting for no apparent reason. It has affected the main stem on the plant, and seems to be spreading to the smaller ones. We don't water it (other than rainfall), and it has been healthy for years. I can't find any evidence of pests. We have had a good amount of rain this spring, but the ground is not overly damp. There is a layer of rotting leaves covering the ground in the bed it is in, so the ground retains some moisture. It is in fairly dense shade, getting little or no direct sunlight (under large Crape Myrtles), but it has done well in that same location for years. Last year, some yard maintenance workers trampled down that area and destroyed much of that plant. We cut back and removed the damaged parts, and the parts that remain seem firmly rooted and the stems undamaged. We have another large Oak Leaf Hydrangea growing about 30 feet away in the same bed, and it is doing fine, though it does get a little more sunlight. The only thing that I noticed that looks unusual (other than the quickly dying plant) is white stuff on the top side of the leaves - but I wonder whether that is normal. It doesn't brush off - seems to be growing with the leaf. I looked on the other (healthy) plant, it is has some of the white stuff - but not as much. What do you think it could be and what should I do about it? I'm attaching three photos. The one showing a relatively healthy leaf is from one of the so-far-unaffected stems of the wilting plant.