my mum asked me for some help with a rhododendron shes had for years. Shes noticed recently some bugs on the underside of the leaves - little yellow things less than 0.5mm in size that seem very flat and sleek in nature. Also, there is a very visible widespread black covering over the topside of the leaves. This however just seems to rub off the leaves if touched leaving a seemingly healthy leaf underneath.
She has told me that spraying it with some insecticide of sorts has not worked and she hasnt a clue what it is or how to get rid of it. Any ideas?