The one spot looks like it, But it could also be staining. The best way to tell is clean your hands thoroughly and then feel the roots, If they are slimy then its root rot. If not it's staining.
If your H2O2
@3% is in a spray bottle. Then spray the infected area directly as well as what Roberts said to clear it up. Once it is cleared then you can start Hydro guard.
**Hydroguard is a bacterium and will be killed with H2O2, Don't use both at the same time either go live or sterile.