If your home has light colored shingles, you may notice black streaking, especially on the north facing side. In areas where there is less sunlight, you will tend to have more dampness. With dampness comes mildew and fungus growth. The best way to prevent mildew and fungus growth is to provide as much ventilation and sunlight as possible. Of course, this is easier said than done in some cases. Try to keep branches trimmed back on trees so that there is as much sunlight on the roof as possible.
There is no quick fix to mildew and fungus short of replacing the roof. I highly recommend that you hire a professional for cleaning your roof. Another option is to check with your local Rochester, or Southeastern MN building supply or hardware store for products that are designed to remove mildew. Make sure you follow all safety and health precautions when using any chemicals to treat a roof. Mildew removal products may affect paint, gutters, and plants, so again, make sure you follow all safety and application instructions.
Another product, which I have installed on my roof, is called Shingle Shield. This product consists of zinc strips that are placed under the edge of the shingles near the peak of the roof. Rain reacts with the zinc to create a chemical that will prevent the growth of mildew and fungus on your roof. Ever notice how white shingles remain white below metal roof flashings? Same concept with the zinc strips. Again, you should consult a professional roofer for installation of this product. It must be noted, that the zinc strips may not remove existing mildew and fungus. You may have to have your roof treated to remove existing mildew and fungus prior to installing the zinc strips.
If you are planning on replacing the roof on your home, you can also look into shingles that have mildew resistant chemicals built right into them. Or have the zinc strips installed with the new roof.