Scott,
I just discovered the exact same condition in my attic above my bathroom. The problem appears to be a combination of inadequate soffit ventilation and the bathroom vent leak into the attic.
I am in the process of fixing the root cause of the attic moisture build up and would like to kill the mold on the underside of the roof sheeting and adjacent rafter areas above the bathroom.
Please respond with any new information.
Thank you,
Pete