Here’s the full question:
I have three young apple trees and they are all suffering from something white and fluffy on their twig undersides. I rub it off but it’s making big scars and sometimes kills the branch. Is this fungal? What can I do to help them?
What you have is an infestation of sucking insects called woolly aphid. It affects apples on dwarf rootstock (not Northern Spy or 793). They attack trees where there’s damage already done to the stems and branches by cicadas. The trees will eventually suffer.
The best solution is to spray the trees with Neem oil during the warmer months and scatter Neem granules on the soil during winter as woolly aphid hibernates in the soil in the cooler months.