Hello,

Because it could be one of several issues, I would have an exact diagnosis done by a lab or a certified arborist. 

It could be spider mites, or one of the needlecast diseases such as phyllosticta. Some years the concolor fir has a more noticeable yellowing of needles as part of the natural needle drop in fall-

https://blogs.cornell.edu/treeipm/2011/10/05/hello-world/

https://www.canr.msu.edu/news/interior_needles_casting_in_concolor_fir

To detect  spider mites, hold a white sheet of paper or paper plate under the leaves and shake the branch or leaves. If mites are present, you will find tiny spider-like creatures drop down and move around on the paper. 

https://extension.umn.edu/yard-and-garden-insects/spider-mites

Of these issues,   your pictures most closely resemble the yearly natural needle loss, except the 3rd one which looks like spider mite damage( the light green color out to the end needles)..

Keep the trees branches dry when you water- this includes your in-ground system. Redirect heads so they don’t spray the trees, as wet branches are prone to fungal diseases. If you must hit the trees with water, water only early in the day so branches dry by sunset.

After you water, check several inches down in the root zone(under the tree branches) of several trees. It should be moist- if not, the irrigation is not reaching these trees. Consider installing drip emitters around the root zones, if you don’t have these on your system yet.

To get the needles diagnosed you can send a sample  branch that shows yellow and still green needles to MSU Plant and Pest lab. Include a picture of the whole tree and closeup pictures as you did here. The fee schedule and instructions are on the website- https://www.canr.msu.edu/pestid/

You can hire a certified arborist, a professional who has taken training in care, diseases, pests and passed certification tests. He/she will come on site and give a complete diagnosis and a plant care plan of all the trees at once. Find certified arborists by zip code here—-

www.treesaregood.org

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