David, this is a plant which is very prone to mildew. There are several in gardens along the road from here which suffer almost every year. I don't believe any of these plants in my street are being treated against the mildew in any way but the plants seem to survive well enough from year to year. I would recommend collecting and destroying the leaves as they fall, if nothing else it'll make you feel you are doing somehing but I believe that climatic conditions are foremost in mildew attacks and spraying is not likely to be much help.