Mold under hardwood?

I had to take up some hardwood flooring, which had been there for 5 years. Underneath I found some 'blackness' on the OSB under the exterior door, and a window. could this be mold?

 

The black is likely water stains because of location near door and window. If there is concern re: mold, disinfect the affected area. Check for possible entries for moisture and seal to prevent. Someone could have left door and window open and moisture entered. Also check to see if door and window are properly sealed around. Make sure water does not leak under door threshold or window sill. Make sure gutters and downspouts are clear and carry water away from the house and that water does not shoot back behind gutters and run behind and down or inside walls.