I know this is an old thread BUT discovered a plant in my garden with this problem back in August and have not been able to tackle it yet. I'm wondering if I should cut every leave off or is it not too late to treat it. I am hoping to visit a garden centre next week to look for a spray if necessary. I do not have the strength to dig it up.
I have noticed a lot more plants affected by mildew this year, most of my clematis have some on, it has never happened before, I think it must be the wet, warm weather this year.
On Friday, this small leaved yellow Hebe, 3ft wide and high was fine, today I have chopped it down as far as I can because it has shed a lot of leaves and the branches are covered in white powdery substance. In my 50 years of gardening I have never seen anything like it. Have any of you any idea what it can be? I just hope it doesn't take my 12 ft Magnolia next to it.