Without seeing the tree it’s hard to be 100% certain, but from your description this sounds very much like mealy bug. Mealy bugs often gather on soft new growth and leaf stems, looking like a dusty or cottony white coating. They suck sap, which is why affected tips can look weak, curled, or stunted while the rest of the tree carries on as normal.

Many varieties of apple trees, and particularly the older varieties such as Sturmer are very susceptible to this problem.

It can be hard to treat and with it producing edible fruit it’s important to be careful what you use.

• Because this is a fruit tree, the simplest approach is to start with physical removal first:
• Once the new growth has hardened off (not soft and sappy), use the hose on a fine mist with strong pressure and blast the affected tips thoroughly. You may need a ladder to reach properly. The key is being really thorough - if a small amount is left behind, it can re-infect the tree again quickly.
• If the problem persists, you can follow with a spray of Neem Oil, which can help suppress mealy bug.

Longer term, keeping the tree strong helps it cope better with pest pressure. Regular feeding is a simple way to support healthier growth and better fruiting:

• Apply Daltons Premium Fruit & Citrus Tree Fertliser from October to Christmas, then start again February to mid-April.
• If you keep seeing fresh “powdery” patches, check closely along the undersides of new leaves and around leaf joints - mealy bugs love hiding in those sheltered spots.