A good alternative to the wire would be an aquarium pipe cleaner. It is designed to fit in small diameter pipes and clean out algae that may be growing just the pipe. It should do the same for any mold that may be growing in your pipe.
The curse of living in a wooded area. A pressure cleaner is a great way to clean your wood outdoor floor. Afterwards a stain sealer is a good finishing touch. This won't cure the dust but will sure look nice. A word of caution... stick with a pressure washer with about 1500 psi, you don't want to blast out the wood pulp.