gapped the decking properly (minimum 1/8")

verified that the gutters do not discharge onto the deck

checked that gutters are not backed up and overflowing onto deck

dryer vent doesn’t discharge directly onto or around deck

pitched the deck properly away from the house for drainage (minimum 1/8" in 8’)

ensured that there is proper underdeck ventilation (latticework only on sides of deck), and that the underdeck area is clear of pooling water

avoided the use of bark mulch around, on or under the deck (bark mulch may be heavily laden with mildew spores)

avoided the use of lawn or plant fertilizer on the deck (such as overspray from lawn fertilizer)

swept the deck every week, especially when pollen is heavy (typically spring and late summer)

See Guide to Mold & Mildew Prevention