Mildew is a fungus that will grow on any organic compound. It likes moisture and warm climates. It feeds on the oil in solvent-based paints or the thickeners in water-based paints. The spores are microscopic and are present in the air around us. They can remain dormant indefinitely and can grow underneath coatings if not removed.
On a cool, cloudy day:
Apply a solution of ½ cup TSP, 1 quart household bleach and 3 quarts of water to affected areas.
Agitate with a scrub brush.
Let stand on the surface for at least 45 minutes.
Rinse off with a garden hose.
Allow surface to dry completely.
Top coat with mildew-resistant paint.