I have some large second growth Douglas firs in my yard that were topped about 20 years ago. The last several years, almost all of them have developed pitch oozing down their sides from up high. What might be wrong with my trees, and what do you think I should do now?
Disease and pest diagnosis is impossible without actually examining the affected plant. However, based on the symptom of oozing pitch you described, these Pseudotsuga menziesii (Douglas fir) could be suffering from one (or more) of the following pests:
For a proper diagnosis you could hire an arborist. The Pacific Northwest Chapter of the International Society for Arboriculture has a directory of certified arborists.
You could also take many photos and a plant sample to a Master Gardener clinic. This is a free service run by volunteers trained by WSU faculty. Clinic locations and times can be found at this website (Plant Clinic Schedule).