My first thought would be to call a paint store. To prevent this kind of thing make sure you don't have any bushes to close to the home. Mildew/mold grows in dark damp places.

I would power wash and as Emma stated ask at the paint store what you could use.

Power washing at this time of year would be tricky, as that can drive moisture into the wood which will perpetuate the problem with mold and rot, and would it be able to dry thoroughly before you paint?

Sorry though, I am not familiar with the products you mentioned, I would ask a good paint store and also post on the painting forum.

Hydrogen peroxide is recommended for porous surfaces but it will take a lot if you mix 50/50 with water in a pump sprayer, bleach is only effective on the surface so it's only recommended for non-porous surfaces.

Definitely don't use a power washer this time of year unless the temperatures are high and humidity is low.

Thanks for all the replies!

I live in coastal North Carolina, so moisture is in the air most of the time. The area I'll be treating before repainting doesn't have bushes or other greenery blocking sunlight or airflow; it's just the shadier side of the house. Since, for now, I'm just patching and painting an older screen porch, I don't think I would opt for power washing. Moviegeek, I may try the hydrogen peroxide idea. If so, I'll post after.

Thanks again!

White vinegar can also be used but it's not as effective as hydrogen peroxide, it does come in gallon jugs and is cheap(non-toxic as well).

http://blackmold.awardspace.com/kill-remove-mold.html