diseased plum trees

Disease / Pest
Symptoms

Plum Sawfly Small holes in the plums, sometimes with a gooey black liquid seeping from the holes. The young plums will fall before ripening fully.
Red Spider Mite Leaves fall earlier than normal and are bronze in colour.
Rust Slightly raised round spots on the underside of the leaves. The spots are rust coloured (browny-red).
Silver leaf Silver coloured leaves turning brown, wood flesh stained brown or purple, fungus growth on the infected branches.
Bacterial Canker Shallow depressions at the base of branches during Autumn which enlarge in spring. An amber like gum may also appear.
Aphids Leaves curl and new shoots are distorted

Any diseased fruit should be removed immediately because they will only attract diseases and wasps.

Aphids breathe through their skin. Spraying with diluted washing up liquid clogs up their skin and causes them to literally suffocate. Their brains are unable to detect pain according to the majority of research, so don't worry unduly about any pain this may cause!