What are you using to top dress? Manure (possibly uncomposted)? Do your trees definitely need it?
Are you sure it's mold and not beneficial fungus?
I'm growing at a very high density with a lot of groundcover and support plants. If I let it become a complete jungle then I have problems with mold, scale and insects. The extreme density can prevent airflow and UV light exposure which keep mold in check. Everything grows healthier when I prune aggressively and remove plants that begin to crowd others and cause problems.
Start by thinning out the existing plants by pruning and removing so you can get some air circulation and UV light penetration. Take a break from the organic matter and switch to a granular fertilizer for now, or stop fertilizing for a while and see what happens. Water less, let the garden dry out a little bit. Maybe wood chips would help.
