Kill grass from tree trunks to one or two feet outside the drip-line. Then mulch, but not touching trunk.
Those already mulched should have mulch and soil pulled away from trunk down to top of first roots. A 2- or 3-inch air gap between mulch and trunk is good enough.
Fertilizer stakes were invented for pine reforestation projects on mountainsides. Fruit tree roots near fertilizer stakes can be burned, but the main problem is that the other roots away from the stakes get no fertilizer.
Fertilize with granular fertilizer, spreading it evenly from a few inches out from the trunk to two feet or more beyond the drip-line. Some of the nitrogen and potassium should be slow release, and there should be lots of magnesium, sulfur, iron, manganese, zinc, and copper. If your soil does not already have plenty of calcium, add some gypsum. A detailed soil test showing all these nutrients would be helpful. Many free or cheap soil tests only show NPK and pH.
Nutritional sprays also tend to be very helpful, and there are many to choose from--- Southern Ag Citrus, Seaweed Extract (Kelp), chelated micro-nutrients, etc.