I appear to have plum rust on one of my plum trees (yellow mottling on upper sides of leaves, brown powder on the undersides) with about 90% of the leaves on the tree being affected. What, if anything, should I do about this? Collins Guide to the Pests, Diseases and Disorders of Garden Plants suggests spraying with zineb. Is this a good thing to do? Can it be treated organically? Is it likely to spread to damsons, cherry plums, sloes and another plum tree growing nearby? Will it reappear next year? Any comments will be gratefully received.