Pin Oak is about 12-15 years old. In error, 3 yrs ago I put a ring of those heavy bricks around then filled in with good soil, planted flowers..yes I know this was not a good thing-now. Last year and this year I started to notice that the leaves remained the new spring green color instead of the neighboring pin oak's darker color. So now I have removed the dirt, transplanted flowers and took one layer of the bricks off & will remove the bottom layer w help of son. I live in McHenry County, northern Illinois...is there anything I can feed the tree or do to keep it from getting sicker and sicker? I do have a lawn service and yes they do spray for weeds and feed..not sure what they use but they try to stay away from plants...so I am told by them. Also a grey covering on bark...hate to lose this tree.
Sounds like you have some damage to the root system with the flower bed and excess soil. I would try fertilizing the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. If you apply the fertilizer just before a rain event you will not have to water. Do this now and again in the spring and this will increase the overall health of the tree. Make sure you do not use a granular product called Weed and Feed near the tree. This product is for grass and contains a herbicide that will kill a tree. Liquid treatments of the grass usually do not effect a tree.
The grey color on the bark may just be natural but could be a fungi. Attach a picture of the bank to a followup question showing the grey and I should be able to tell if it is a fungi or not.