WOOLLY APHID
The woolly aphid (eriosoma lanigerum) appears in the spring on the bark of some fruit trees - it is common around bark which has not been cleanly pruned. The aphids are covered in a waxy substance which makes them look like a fungus or mould. If you rub your finger over them, the aphids will be crushed and wet, which is the proof that it is not mould. The waxy coating makes them difficult to treat with sprays.

Where the pests are ignored, the bark will develop lumps (noticeable in winter) which may split in frosty periods, leaving the tree open to apple canker

How to Treat Woolly Aphid
If the aphids are noticed early, simply paint them with methylated spirits, or scrape them off individually. If they are ignored and larger areas are infected, spray with derris (available from most garden centres). Failing this, cut the resulting lumps out from the bark.