Pest or Disease?

[DebsC]

The symptoms start at the tip of the leaf as a little bit of a lighter green than the rest of the leaf. Gradually and sometimes not so gradually, the tip yellows and it goes down the rest of the leaf but doesn't affect the bulb.

The surface of the leaf is lightly pitted like a tiny insect might have nibbled on it, but it's all over the leaf so I wonder if a tiny insect could do this much damage?

Got any ideas?