Mildew:

Black, grey or brown areas on the surface of paint or caulk.

Possible Causes:

• Forms most often on areas that tend to be damp, and receive little or no direct sunlight (walls with a northerly exposure and the underside of eaves are particularly vulnerable).
• Use of a lower quality paint, which may have an insufficient amount of mildewcide.
• Failure to prime bare wood before painting.
• Painting over a substrate or coating on which mildew has not been removed.