Quick Answer Those black streaks running down your siding aren't dirt, mold, or soot - they're a living bacteria called Gloeocapsa magma. It travels by wind and rain, feeds on the limestone filler in asphalt roof shingles and the minerals in vinyl and brick, and spreads fastest in humid, shaded areas. Regular washing with the correct soft-wash solution kills it at the root, while pressure-washing alone often just spreads the spores around. What Are Those Black Streaks, Really