Mold inspectors are getting a work out after this wet spring and summer, high humidity and shade. Mother nature is providing mold and mildew a near perfect ecosystem in a place many homeowners never consider–on the siding.
A good rinse with a highly diluted bleach solution and your garden sprayer will probably take care of the problem, though you might need to put in a little elbow grease.
Just be careful if your siding is on the geriatric side, because if it contains lead or asbestos, it may be degrading and need professional attention or replacement. Lead poisoning –and for that matter, asbestos poisoning–are a very real concern, and so is the dirty (read contaminated) water running off of degraded siding.
If you’re planning on handling this yourself, the EPA provides guidelines regarding avoiding lead.
http://www.epa.gov/lead/pubs/renovaterightbrochure.pdf
If this is your situation too, and you’re in California, you might want to give us a call and see about getting an assessment from Byebyemold.