Shelley,

You've had an assessment done by experts, which I am not, I just love trees. I've met Ken and toured his garden in Michigan. I consider Ken a mentor and have followed his posts from the trees and conifer forums for a decade. He knows his stuff! I don't know that we have much rhizosphaera needlecast here, but it's often the cause of declining Colorado Blue Spruce in Michigan and in the east in general. I think the arborists you had out would have identified needlecast if the tree had it.

I would not fertilize until you've done a soil test. I'm linking Colorado State University's soil test info.

Soil testing CSU

While there is a school of thought that a little fertilizer won't hurt, it might be a waste of money if it's not needed. You can also do some real harm if you over-fertilize a stressed plant.

What the soil test won't tell you, unfortunately, is if chemicals were applied in the area. Have you or any of your neighbors sprayed chemicals or applied weed and feed around the tree?

I'm suggesting a soil test because it will give you a report that explains your soil composition. It will tell you what nutrients you have and what you're lacking. That narrow things down for you, so you can focus on adding only the nutrients your soil needs. If you don't want to send a sample to CSU, your county extension may be able to provide you with containers and instructions. I'm pretty sure your extension will send the sample to CSU anyway as that's the state lab. I'd recommend the routine + SAR test just to make sure you don't have too much sodium. I think it's $38 well spent to know what's in your soil.

I do have some other thoughts. 4 years ago would have been the year after our 2013 flood. Our landscapes got inundated with moisture which stimulated terrific growth. The following year, 2014 is when we had the flash freeze. In the fall of 2014, leaves stayed on the trees and didn't turn color till very late in the season as night time temps stayed warm. The trees were still actively growing and hadn't slowed down for winter like they normally would. It was 65 or 70 degrees one day and the next day it got to 15 below. I don't know if you remember, but it killed some trees (especially fruit trees) outright and damaged many others, all across the region.

It's possible that what you're seeing with your tree is residual damage from the flash freeze, it just took a little while to show up. Once the bottom branches died and you cut them off, it exposed the soil at the base of your tree. What used to be shaded was now exposed to sun, which dried the area out. Blue spruce like water and It's been pretty dry the last couple of years. It is recommended that you water trees over the winter once a month if it's above freezing and we don't have snow cover.

The arborists have said the tree is basically healthy. I would take them at their word. Did they or you dig around a bit to check soil moisture around the base of the tree? My suggestion would be to cut off the remaining dead branches, give the tree a thorough soaking, (hose at a trickle for hours at the base of the tree, moving it around the dripline every hour or so until water runs off), then apply 2 -3 inches of mulch at the base of the tree, keeping it a few inches back from the trunk of the tree. See if that stops the progression of die back. It looks like we are in for a dry year, so check your soil moisture and water when needed this year.

I didn't mean to go on so long! I hope my random thoughts help.

Barb