The deal is that fertilizer added too late in the season causes a flush of growth that depletes the coldhardiness of the plant. Growing tips are carbohydrate sinks. Carbs are the antifreeze that makes the buds resistant to cold. The tree looks fine. I would wait until next year.
Give it a pass this year.
In the colder parts of Ontario, shoot for 12" of growth per year. In Iowa, you can shoot for 24". In either case, try to get the fertilizer on the ground before the dandelions bloom. Once the tree is the size you want, you can back off on the fertilizer so the terminal growth is half the values listed.
