How long have you had the plant, is it in a pot or the ground? Check the stems for scale insect and the leaves for other infestation, if in a pot, turn it out to check the condition of the rootball and whether there is any infestation in the pot. Otherwise, spray with a treatment for mildew.

We have had the plant since December and it is still in a pot. I have checked it for scale and cannot see any signs. Thanks for your thoughts.

Check the roots to see if they're healthy - vine weevil can destroy the roots on plants over winter.