Black streaking on a Loudonville roof is Gloeocapsa magma, a hardy algae that feeds on the limestone filler in asphalt shingles. It starts on the shaded slope and spreads across the whole roof over a few seasons.
Loudonville's dense mature canopy shades whole roof planes for most of the day, and cedar and architectural shingle roofs under those trees grow moss and lichen along with the usual algae streaking.
Pressure washing a roof strips granules and shortens its life. We use the low-pressure soft wash the shingle manufacturers actually recommend: apply, dwell, rinse gently. Streaks are gone the same day and stay gone for years.