TSP, hot water, and a household cleaning brush should do it. A pressure washer may help too but be careful that the high pressure doesn't dent the siding.
The TSP is toxic stuff so wear eye protection and gloves. A white, long sleeve shirt (which will keep you cool in the hot sun) and long pants are good too. Read the info on the TSP container for more.
My best to ya and hope this helps.
PS: Oh yes, be careful not to get any elbow grease on the siding. ;-)
© 2016 Renovate Your World LLC