I noticed this white almost chalk like residue on my corkscew hazel the other day.
It is powdery mildew.
Caused by a variety of fungi including Oidium, Uncinula & Sphaerotheca; the powdery chalk like residue sits on and clings to the top of the leaf.
It’s sister downey mildew clings to the underside of the leaf and is damp and fluffy to feel… So as not to confuse the 2, remember: powders could not sit on an underside [as simple as that sounds – to the non horticultural C.S.I. diagnostics team it is very important 😉 ].
Back to it…. One must remember that this is a fungal problem. And spores are spread by wind, rain or even plants rubbing together. Powdery mildew likes a dry site and fungi usually grow in areas where it has little chance of being disturbed. So, whilst it can be sprayed/ treated chemically… this will solve the immediate problem but, the chances are the disease will return as the conditions/ environment have not changed. My methodology is to remove all of the diseased material; then wait if possible ’til the off season and move the plant to less enclosed spot.
The reality is, one should also remember that this is not a bacterial disease of the plant so whilst photosynthesis is affected; and therefore fruit/ seed production – the disease is not as such detrimental to the plant.
Chemcal treatment is usually done via the use of a translocated/ systemic insecticide and fungicide mixed as most insects are disease vectors. Make sure [please] you have a seperate applicated sprayer to the one used for herbicide 😉 That said, I prefer working with nature where possible and would always first recommend the biological control first.
While I’m here… if you are spraying it can leave a white residue…. don’t confuse the two 🙂