White fluff on your apple tree is not a sign of a fungal infection, but more likely an infestation of woolly aphids. We explain more here.
Aphids in fruit trees - Agricultural Biology
Woolly Aphids Observed And Reported On Social Media
Woolly aphid on apple trees - Sustainable Macleod
Aphids - Integrated Pest Management
Woolly Apple Aphid, WSU Tree Fruit
Woolly aphid - Suffolk Fruit and Trees - The Fruit Tree Specialists
Woolly aphids on a Beech tree
Emergency authorisation for woolly aphid control
Woolly Aphids in the Garden: Prevention and Removal
Woolly Apple Aphid