"You do normally need to fertilize apple trees only once per year (if that). It the tree is putting on good growth, it may need no supplemental fertilizer. You can apply the fertilizer from late winter through early summer. Avoid fertilizing late in the growing season because that can spur late season growth that won't harden off before freezing weather hits.
Although you live in the mountains of North Carolina, you can still follow the gardening guides for the South."