Dying Fir Trees

Hi, new here.  I live in Western Washington.  Have been on the property for two years.  We have lost about ten conifers in the last year or so.  They were beautiful and healthy looking.  Suddenly the needles will begin to turn brown, beginning at the base of the tree, moving up to the top within three days.  Within five days from the beginning of this all the needles on the tree will be gone, lying on the ground.  The needles look a healthy green in the beginning, rapidly turning brown, than falling off.  This occurs spring, summer , and fall.  There are no herb or pesticides being used.  These are naturally occuring, established trees on the grounds.  The dying trees range in size from ten feet high to well over 30 feet tall.  It appears to occur in groups-one will die, than another, then two more, etc.  On the one tree I pulled up, I have not seen insects or rot of the tree above ground, although the roots appear to be dead, perhaps rotted, certainly not healthy with light roots branching out.  I suspect fungus (?).  What can I do?  Again, nothing new has been done to the grounds, these trees get natural rainfall (plentiful), good drainage, and all the other things a healthy tree would need.  Thank you!