Question
Hi, My Miniature weeping willow has developed a yellow fungus on the leaves and the wood, it looks powdery and it is in small spots and patches. It is making the leaves curl up. Do you know what this is and if so how to treat it? Hope you can help me, Many thanks, Mary Roberts.

Answer
It sounds like powdery mildew. This is a fungus that infects a large number of different plants.
On some plants, powdery mildews can result in significant damage. Thus, fungicides must be used to achieve acceptable control. For best results with fungicides, spray programs must begin as soon as mildews are detected. Spray on a regular schedule, more often during cool, damp weather. Use a good spreader-sticker with the fungicides. Be sure and cover both surfaces of all leaves with the spray. Fungicides generally recommended for powdery mildew control include: Triadimefon (Bayleton, Strike); Triforine (Funginex), Thiophanate-methyl (Cleary's 3336, Domain) and Propiconazole (Banner).
Spray the foliage and stem with one of these fungicides. Your local nursery will have a fungicide for powdery mildew.