mold/mildew on sheetrock under window

We bought a 1950's house, old windows still in part of home. We found a little mold/mildew at the bottom of the sheet rock near the floor under where the window is hung. Wondering if it could be window seal breaking down, or what other possible causes. There's no water/plumbing near where the problem is. Thanks for your help.

 

You should probably remove the sheet rock to determine the cause of the leak and extent of the damage.

 

Is there excessive condensation on the window? The drywall had to get wet somehow.

 

No. The window doesn't appear to be wet at all. The mold/mildew may have been there for years. As I said, the house is 57 years old and so are the windows. The sheetrock is not even wet. Of course, if it were rain coming in, I wouldn't know if that was the cause because we are in a drought. But it's only one little spot. Doesn't appear to be spreading. It's almost as if the problem is old, because it's turned a black color, not green.

 

I might be going overboard here, but I'm concerned about what's on the other side of the sheetrock.

 

Is the mold above the baseboard on the sheetrock, or have you removed baseboard and found mold behind it? The reason I ask is that I've found that its pretty common to find mold behind baseboard due to air infiltration (usually winter only) when cold air -coming under the sill plate or from inside the wall cavity- is meeting warm moist air. Something similar could be happening in your home periodically when the conditions are favorable for it. It might not have anything to do with the window, other than perhaps indicating poor insulation or poor air sealing in that area.

Only other thing I could think of is that maybe someone left the window open during a big rain and it's since dried out. Sometimes people will caulk on storm windows (including sealing the bottom sill expander shut) and then leave the storm windows open during a rainstorm, only shutting the prime window. They don't realize that caulking the bottom of the storm window is creating a dam for water. Water then goes the only place it can... down the ends of the sill and into the wall cavity. Usually these windows will develop huge rotton spots under the bottom corners of the storm window- hard to see unless the storm window is actually removed. But a few pokes with a screwdriver will usually tell you if the sill is rotton or not. I kind of doubt that's the source of the problem here.