I have a row of mature eastern pines in my yard and 5 years ago one browned and died. I couldn't discover any reason so I had it cut down and the stump ground up. Took out as much wood as possible and planted a new tree.

Three years after that, this tree now ~10' tall, browns and dies as well. I don't see any reason but I did notice when I cut it down that the wood seemed unusually lightweight and that the interior wood seemed to have a gray infestation. Is this gray stuff something to be worried about in regards to the mature pines nearby?