Audi gave me a 1m piece of the diesel hose for free, so I replaced one of the old hoses and did a post mortem on it, split it lengthways. The part where it started to decay is discoloured with a faint white stain (to the right). It was still holding vacuum though and not completely decayed yet.
The dust is actually produced by the fabric braiding, it decays first and falls apart when you touch it leading you to think its the rubber underneath thats falling apart in your hands, but its usually just the fabric, if you still have vacuum (test by disconnecting one hose, listen for the escaping air) the hoses are still ok.
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