I am almost certain that the two corners you are referring to are the northeast and northwest corners, where the house receives the least amount of sunlight. Without the sun, the drywall in those corners is colder and that reacts with the house's normal humidity, causing the mildew and excess moisture. The first thing that you should do is clean the wall using bleach to kill the mold. Then you should determine if there is too much humidity in the house or if the affected area is just too cold. The easiest way to check this is to look at your windows. If they are foggy most of the time, then there is likely too much humidity and you will have to better ventilate the house. If the windows are clear, then the problem lies with the temperature in the affected corners, and you will have to add insulation over the affected area to warm it up. One easy way to do this is to open up the soffits and put rigid foam insulation on the outside of the house over the affected area.