Black streaking on a Halfmoon 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.
The Mohawk river corridor through Crescent and Vischer Ferry keeps humidity high, and newer Halfmoon subdivisions with young landscaping still get streaking on the north slopes within about six years.
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.