This plant can get grey mould (Botrytis). this is what the RHS say for this:
Non Chemical control
Remove dying leaves, buds and flowers promptly
Do not leave dead plant material lying around
Reduce humidity by improving ventilation and do not overcrowd plants
Chemical control
No fungicides are approved for use against grey mould by amateur gardeners. Products containing plant and fish oil blends (Vitax Organic 2 in 1) may be used, but are not recommended by the manufacturers for grey mould control and are unlikely to have much impact. Use of other fungicides to control other disease problems may give some incidental control of grey mould, but this is not guaranteed by the manufacturers.
Sorry it is not much help. It may be worth getting rid of the badly affected one and taking cuttings from the good one?

move the pant to a more moisture retentive place i had one never got had any probs with mildew was in a very shady place in back garden where the soil never dries out it loved it and got bigger so i moved it to the front in the sun ......it disappeared first year never to seen again! if a plant likes where its growing dont move it! if its unhappy try moving it.....try splitting it then you got 2 chances