There are 2 chems I use,...
Bleach is great, fast acting and effective, BUT it can grey the timber if you apply it too strong or leave it on too long.
Te second option is sodium percarbonate (Oxy bleach) which is peroxide based. It does the same job (a bit slower) but is less likely to remove the woods natural colour. You can still do damage if you leave it on too long, but it takes a lot more time to cause damage. Its also eco friendly as it degrades into water & ash after about 4 hours.