Q.Fertilizing a cherry tree while treating it for black knot fungus
We are trying to save a cherry tree overcome by black knot fungus. We cut away as many of the galls as we could without cutting the tree down and have received advice from the local extension office about best fungicide to use.
My husband wants to fertilize the tree but I want to be sure that this will not interfere with our effort to get rid of the fungus.
Can you advise?

Though early spring fertilizing can be done, I don't like to fertilize a tree into health.
You may want to have a soil test done and see if the nutrient levels are low in your soil.
If then needed, you can fertilize appropriately.
https://www.gardeningknowhow.com/edible/fruits/cherry/when-to-fertilize-cherry-trees.htm