Answer: Based on your description I am not certain what happened to your tree, although the symptoms sound a bit like possible frost damage due to the oscillating temperatures we had this spring then followed by dogwood anthracnose which tends to badly affect stressed trees and can eventually kill them outright. Here is some information about the anthracnose, you may have to cut and paste the complete url into your browser to make it work correctly.
Fertilizing would not help a stressed tree, so the spikes if anything could have added to the stress.
You might want to consult with your Rutgers county extension and see if they can give you a more specific diagnosis and based on knowing that, determine if the tree can be saved. If a chemical control is needed, they will have the most up to date information on what to use and how/when to apply it for best results.
In the meantime, the best things to do are to clean up all the affected leaves and dispose of them in the trash. Then keep the soil evenly moist like a wrung out sponge (not sopping wet or saturated, just damp) and mulch with two to three inches of organic mulch spread out in a flat layer over the root area, allow several inches of clearance between the mulch and the trunk of the tree.
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