Like Charlotte, I think it could be the new growth. But if the leaves are definately yellow as opposed to pale green, then it could be magnesium deficiency. You could try giving them a liberal spray with either Epsom Salts or liquid seaweed. Neither will do any harm and who knows, it might even cure it.
I heard last year about some kind of box disease but IIRC that made the leaves go brown, not yellow.
G xx