Try bleach in a water solution and spray the siding with a hose end sprayer set up.
Power wash with a washer that allows you to put in chemicals for this purpose, but, be care ful not to damage your shingles and do not spray up ward. Spray downward.
If this does not work, check out Woodscapes by sherwin williams. This is made for cedar or wooden shake type siding. Its a stain, paint that has to be applied by sprayer. It can be by hand, but it takes along time. Cedar soaks up paint.