apple tree

This site tells you all about woolly aphid Sunshines, if that is what it is.

http://www.gardenaction.co.uk/techniques/pests/woolly_aphid.htm

John.

Wash them off with a strong jet of water is one option. Add some horticultural soft soap to the water, it helps no end.
They would stop it fruiting in the sense that they are sucking the life out of the tree, but there could be other reasons too. Get rid of them first and then see what happens.

I hope the treatment works! I had an apple arch made of two smallish apples. unfortunately it got woolly aphid a couple of years ago. We tried all sorts of things to get rid of it but it just multiplied all the more. We have cut the arch down now. It started after pruning and grew on the cut ends of the branches. I hope you are luckier. It didn't actually stop it fruiting.

We had a bad infestation on my M.i.l's tree which was huge. We managed to remove the aphids with soap and power washing, took a few goes though and we did have to do neighbours tree as well, even though it had no signs of them. They over winter as babies and eggs, so spray with Winter Wash about now too to kill them.