Q.Douglas Fir Evergreen Trees turning brown
I have had 2 Douglas Fir evergreen trees die on me over 2 years. Whatever is killing them it only hits one at a time. They are in an open area of my yard, but next to a tree line. I live in NE PA. They turn brown slowly until they are completely dead. We did a mite test with paper and did not see any mites. What could be killing these trees?

I have had 2 Douglas Fir evergreen trees die on me over 2 years, whatever is killing them it only hits one at a time. They are in an open area of my yard, but next to a tree line. I live in NE PA. They turn brown slowly until they are completely dead. We did a mite test with paper and did not see any mites, what could be killing these trees?