The best mold and mildew remover is still regular unscented bleach and water, mixed 50-50. This will kill it all. Plus, you can put it into your garden/bug sprayer.
To protect any plants use cardboard on the ground while spraying and wrap larger plants with thick 3 or 4 mil plastic. Simply spray on the bleach and respray to keep wet for 30 minutes then wash. The best thing is the bleach evaporates, where as other chemicals might stick around and be a problem for the homeowner and their landscaping.
An alternative is injecting an environmentally friendly soap into the power washer, something like Simple Green, then rinse. Wash with this first, then apply the bleach to kill anything remaining. No rinsing is required after the bleach.