I agree it was probably the fertilizer. Not so much the nutrients themselves, per say, but the salt index. With that, I'd approach it as a salt-remediation project, which isn't easy or guaranteed, short-term, but I'd try if it were my tree. Before you start, get a soil test. Here is where I am coming from with the recommendations...
I did an appraisal for some clients where the township highway garage allowed road salt to wash into their woods. I searched for some options to help them. I noticed on a product called Root Magic that I had for other uses it said something about salt damage remediation. I called the manufacturer. They asked where I bought my product - I buy it at PSP Enterprises...the manufacture and suggested D'Wight at PSP as he was probably the best resources. Not the best website...but D'Wight is very helpful and knows his stuff:
http://pspenterprises.us/
I also called a very well-respected soil scientist in this area and he referred me to Don Burgess at A&L Great Lakes Laboratories:
https://algreatlakes.com/ Use them for the soil test and then ask talk with Don about your results....actually call first and ask what he wants you to test. I'm guessing 2-3 different areas and at least 2 different tests.
Here was D'Wight's recommendation to me:
1. gypsum helps tie up the salt, depending on depth of salt penetration into the soil enhances leaching process
2. irrigation will help with the leaching process.
3. organics (Root Magic ) humic and fulvic acid ) help restore the beneficial soil microbes and promote healthy root and plant growth.
4. Bio-Remedy helps detoxify soil and create a healthy soil ecosystem providing all kinds of benefits to soil, plant and other microbes.
Suggestions:
Spread gypsum,
treat with 2.5 gallons of Root Magic + 2 lbs of Bio-Remedy per Acre Spring, 4 weeks later and then again in the fall ( 2 seasons) Your cost-in-use is $338.00 per treatment
if possible, irrigate when needed to maintain leaching process of Sodium,
When replanting use a true organic fertilizer (low sodium or salt index, Assure 5-5-5 with microbes )