Mould/bugs on tree stock

Morning all
I’ve got this nasty white stuff on my matthiola arborescens (tree stock). Have sprayed it with Roseclear, but doesn’t seem to be working - any ideas??
It’s just about to come into full flower, so didn’t really want to decimate it by removing all the affected leaves and burning them.
But I will if I have to - there’s always next year (we hope!)