2 Answers

Could be the dry rot fungi, be very careful when treating this kind of problem, it can spread very quickly, some info here...http://en.wikipedia.org/wiki/Dry_rot

Residue from the mild and fungus. Clean it up. 

Things You'll Need
Cloth
Sponge
8 tbsp. trisodium phosphate
1 cup chlorine
1 gallon water
Mildew-resistant paint

Instructions
1
Apply a mild alkali, such as trisodium phosphate, to a cloth that you can dispose of later. Rub the cloth on the mildew on the wood and apply pressure to the surface. Wear rubber gloves when cleaning off the mildew.

2
Rinse the wood with clean and cool water, using a sponge. Only wet the area enough to remove any excess mold or dirt.

3
Let the affected area dry for at least two hours. If mildew is still present on the wood, repeat the process until you have it all removed.

4
Purchase a mildew-resistant paint and apply it to the wood. You can find small amount of paint to use at your local hardware or paint store. Apply a thin layer and then add an additional layer once the paint has dried.

5
Use a solution of eight tablespoons of trisodium phosphate, one cup of chlorine bleach and one gallon of water to apply to the wood if the mildew returns after you've applied the paint. Apply an oxalic acid solution if a mildew stain returns persistently. http://www.ehow.com/how_5542373_rid-mildew-wood.html