These trees are badly stressed. Deep root fertilization will not address that and is not any more effective than broadcasting granular fertilizer evenly over the entire root zone and watering it in. I would recommend getting a soil test to determine actual nutrient levels in the soil and to assess the pH of the soil. If the pH is too alkaline, the trees will not be able to adequately take up nutrients. The soil test will determine how much fertilizer, if any, to apply and whether corrective action is recommended for pH. 

Can you provide details on how the trees have been treated for needle cast? It would be helpful to know the name of the product used (or active ingredient), the rate applied, how it was applied, and the approximate dates it was applied for each of the past two years. We generally recommend spray coverage of the entire tree with the appropriately labeled product once when the needles are half expanded and a few weeks later once the needles are fully expanded. Under certain weather conditions, an additional spray may be needed later on. 

The best solution is to determine what factors are stressing the trees. Good information on evaluation and treatment are here:

 https://www.canr.msu.edu/news/help-me-save-my-blue-spruce

Given the large number of trees, I would recommend you contact the MSU Plant and Pest Diagnostics Lab. They can complete a complete plant health analysis, examining soil and plant samples for evidence of pests and diseases. You may contact them at: https://www.canr.msu.edu/pestid/

Information on soil testing:

https://www.canr.msu.edu/news/soil-testing-returns-through-msu-extension