Now that is a new one on me. Any insects on the leaf?

Hi, if you lived in a tropical or sub tropical area, I would say that you had an attack of the scale insect Dactylopius coccus, commonly known as the cochineal insect, but as you're in Scotland, that isn't very likely, so I'm sorry I can't help you, Derek.

Nothing in any of my books either.

thank you both for trying. I will see if it develops into insects although it is worrying not knowing whether or not it can spread to other plants in the garden.

Cocchineal scale only lives on Prickley Pears and Chollas, anyway. I'm thinking that yours might be some kind of rust fungus.

Which Crab apple is the leaf taken from? The only one I know which has leaves like that is fairly rare. We have one, cannot remember the name now nor find a label.

Tug I looked up cochineal scale. Cocchineal is extracted from the dead females by pouring hot water on them . The live ones seem to be gray. The only rust I could find affecting crab apples is cedar apple rust. it does not look anything like this from photos I have seen.
Owdboggy I inherited this tree from the previous owners. It produces tiny dark red apples, 1cm in diameter. it looks like the one shown in this link. https://en.wikipedia.org/wiki/Malus#/media/File:Purple_prince_crabapple_tree.JPG

The Missouri Botanic Garden web site shows crimson stains virtually identical to yours as Cedar Apple Rust.

I did look at that but the infection on crab apples is described as orange yellow on the upper surface of the leaves. However on hawthorn trees in the photograph shown, it is pink. On the leaves infected on my tree it starts on the underside and is bright pink. I fear it is rust and from the info given I think the tree will need to go. Thank you for taking the time to find out what it is. We cannot take the tree down, it is too big a job for us. I will try and get in touch with the County Arborist tomorrow and get more advice. I will let you know how I get on.