Black mould

This year my Rhodies and Pieris are covered with a nasty black deposit all over the leaves. This a FAR worse than I have ever seen here before and the shrubs are suffering badly because of it.

I have just sprayed a bug killer and Neem oil emulsion over them. Do you think this will work and if not, what else should I do.

Is the cause a whitefly leaving sticky deposits that then attract this black mould?

Any suggestions to deal with the problem are welcome.