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Yes, growing grass under and around Pine Trees or Spruce in your case is a difficult task.
You are battling acidic soil, water competition, lack of sunlight and all the needles dropping from the trees.
This link has some great information to help you get started.
https://www.thespruce.com/grass-doesnt-grow-under-pine-trees-2152742

Air circulation can also be a factor. I can attest to this. I have an area of my yard that gets good afternoon sun, but is in a sheltered area. I have Powdery Mildew in the grass.
Here is a link with information for you.
https://www.gardeningknowhow.com/lawn-care/lgen/powdery-mildew-on-grass.htm

This article will help you.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/reversing-soil-salinity.htm

Here is a link with information on weed control.

http://www.clemson.edu/extension/hgic/pests/weeds/hgic2314.html

You may have cut the lawn a bit short from a length of 6", but my first thought is urine from animals. Do you have a dog or cat? If not then possibly a neighbour's cat "leaving you overnight messages" If not cats then perhaps a squirrel or even a fox, again coming during the hours of darkness.

Here are some links that may help you compare the damage you are seeing to some examples in the articles. You may want to contact a local lawn service to see if they have seen this damage in other areas in your region.

http://www.extension.umn.edu/garden/yard-garden/wildlife/whats-digging-holes/

https://www.gardeningknowhow.com/lawn-care/lgen/lawn-and-garden-holes.htm

I also might suggest you contact your local County Extension Office. They may have information on your local area and the common lawn pests. Here's a link to help locate your nearest extension office: https://www.gardeningknowhow.com/extension-search/