From the symptoms you describe, it is most likely woolly aphid, not a fungus. Woolly aphids look like little clumps of cotton wool on the stems and in branch forks. If you wipe the fluff away, you’ll usually find small pale aphids underneath, clustered close to the bark and in crevices.

What to do

The most effective clean-up is in winter, once the tree is leafless. Apply lime sulphur over the whole tree (making sure you hit all the cracks, forks and branch joins). One or two applications over winter is commonly recommended for this kind of infestation.

After that, it’s worthwhile following up with a good quality horticultural oil at winter strength, which helps smother any remaining insects tucked into bark creases.

A few extra tips that help:

• Baking soda won’t control woolly aphid, so it’s normal that it only “cleans” the surface temporarily.
• Once you’ve knocked it back, check the tree regularly through the growing season so a fresh colony doesn’t build up again.
• Don’t worry - woolly aphid is very common on apples, particularly in warmer northern areas

Photo Credit: Woolly Aphid image by Stephen Cresswell  /  bugguide.net