Try bleach or mildew spray and scrub well with a short haired stiff scrubbing brush or a scotchbrite pad. Rinse well immediately after and remember to reapply tube protector. The above will leave the tubes horribly unprotected otherwise.
If all else fails, try Acetone. Be really, really cautious though. Go in too hard or too long and it'll take the surface off of the tube material. Been there with sun eaten pvc dinghies.
