Question
i have noticed that my magnolia and hydrangea have a silvery slightly powdery
dust on their leaves i have recently lost a willow to this i think as it died for no
apparent reason

Answer
Sounds like a disease called powdery mildew. A powdery, fluffy white to light gray-colored fungus growth on succulent stems, leaves, buds and flowers is the characteristic symptom of powdery mildew. Young plants and actively growing shoots are more severely damaged than older plants, leaves or branches. Infected leaves may be dwarfed, curled or deformed.

To control use one of the following fungicides according to label instructions:

Propiconazole (Banner Maxx) is registered for control of powdery mildew and numerous other diseases on ornamentals.

Myclobutanil (Systhane, Eagle, and Immunox) is registered for control of powdery mildew and numerous other diseases on ornamentals.

Sulfur as a spray or dust as needed. Sulfur may cause some plant injury if applied when air temperature exceeds 90 F.

Triforine (Funginex) is labeled for use on roses and several other ornamentals. Follow manufacturer's instructions on the label. This product is only available in small packages.

Triadimefon (Bayleton and Strike) is now registered for use on a number of ornamentals.

Fenarimol (Rubigan) is a locally systemic fungicide for the prevention or therapeutic control of powdery mildew of field or landscape grown ornamentals.

In some situations copper fungicides will provide control of powdery mildew.

Check with your local nursery store for a fungicide for powdery mildew on trees and shrubs.