Trees are causing mold and mildew on my roof shingles. What can I do?

Your stains are probably caused by the airborne spores of a blue-green algae known as Gloeocapsa Magma, which is common in the South, rather than mold or mildew. While doing little harm to your shingles, the black streaks are unsightly. Algae stains can be removed with chemical cleaners, though they usually return over time. Repeated use of harsh chemicals (such as bleach) or pressure washing can damage or shorten the life of your shingles. One option for preventing stains is to install strips of zinc or copper -- which are toxic to algae -- along your roof below the ridge. Molecules of the metal then wash down the roof each time it rains to keep new algae from forming. A better, long-term solution is to specify algae-resistant shingles, which have copper granules embedded in them, the next time you have your roof replaced.
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