This Sugar maple tree had a beautiful canopy in 2022, and is located behind one of our barns. I noticed the same degradation over the same time period with 1 other sugar maple in the same fence row. What generally causes gradual crown dieback without any physical damage? 

We have many large, 100+ year old maples nearby and Im worried that what ever caused the decline in this tree will spread. 

The bark looks very dark with thick white blotches, at first I thought it was lichen but now thinking it might be something else. 

Picture below is the canopy in Aug 2024