Stripper make good cleaners
A sodium hydroxide based stripper can be a very good cleaner. The SH will kill the growth, while the surfactants and water agents will help get a good clean, while using fairly low pressure. A sodium hypochlorite (bleach) solution can work great too as has been said above. (dwell about 15 min.) Hot water is definitely a NO-NO! It can raise the grain. Keep your bleach strength to between 1% and 3%. Let dwell about 15 minutes. Soap helps the bleach with the clean so don't forget to use some. (You don't wash your clothes in bleach alone) I always liked the stripper over a bleach solution because bleach can't penetrate wood so spores and roots don't always get fully eliminated. Always follow with a neutrlizer (Brightener).