Re: Mouldy Petunias.
Pauline, it sounds like what is called Powdery or Down Mildew to me.
Pansies often suffer with this too.
Strangely, it often starts when plants have been have got too dry at the roots, and as Meg said, they need air round them as well.
Here's just a short exert for you:
Powdery Mildew: Powdery mildew doesn’t need soaking wet conditions to thrive, but often appears where plants are spaced too tightly, obstructing airflow.
Look for white, powdery spots of spores that spread or cover leaves and flowers completely. Powdery mildew can be treated with neem oil, but you should also correct the conditions allowing this disease to get a foothold."
Unfortunately I have never been able to cure my Pansies and usually end up pulling them up.