Question

Leaf   We have a new small pussy willow tree which seems to be healthy and sprouting new growth but it is being eaten badly by caterpillars and the underside of a lot of the leaves is covered in a yellow powdery substance. Is the caterpillar eggs or could it be a disease?

Answer
This is a disease called a Rust. Yellowish Spots And Dark Pustules On Leaf Undersides due to Rust. Some rust diseases that are caused by fungi attack willow leaves. They are signaled by lemon yellow spots on leaf undersides, that later develop into spore-bearing pustules. A severe infection will cause the willow leaves to drop. Although rust infections are not considered serious, they can defoliate young trees. Collect and destroy fallen leaves and prune out affected branches. Spray willow foliage with a copper fungicide. Check with your local garden type store for a fungicide containing Cooper. They will also have an insecticide that can be used to kill the insects eating the leaves. Orthene is one that works well but there are others.