What Causes Black Streaks on Arizona Stucco
Black or dark grey streaks on Arizona stucco appear in two patterns: vertical streaks running downward from debris accumulation points (gutters, trim, window frames) — typically algae fed by slow drainage; and horizontal or irregular dark patches on north- and east-facing walls — typically mold or mildew from monsoon-season moisture retention. Both are living organisms requiring treatment chemistry, not just cleaning.
A common misconception is that dark stucco streaks are hard water staining or rust. Hard water staining is white to cream-colored, not dark. Rust staining from irrigation is orange to brown. True dark streaks are almost always biological. Diagnostic: if streaks get darker and more extensive each monsoon season, they're biological. If static in pattern and color, they may be mineral-related.
Safe Black Streak Removal from Arizona Stucco
Soft washing is the correct method for biological streak removal from Arizona stucco. A biodegradable algaecide/fungicide solution is applied at low pressure (under 400 PSI), allowed to dwell 5–15 minutes to kill the organism, then rinsed at low pressure. The chemistry dissolves the biological growth from the stucco surface without the erosion risk of high-pressure washing.
Never paint over biological streaks — painting over active growth traps the organism beneath the paint, causing bubbling and staining through the new coat. Always treat and remove completely before any repainting project. Post-treatment antimicrobial application inhibits regrowth for 3–6 months.