My sturmer apple tree has a powdery appearance on some of the growing tips. The leaves affected aren't thriving whilst the rest of the tree where unaffected is doing well. What is this, and how do I deal with it please?
It’s hard to be absolutely sure without seeing the plant, but from your description it does sound like an insect called mealy bug is the cause. 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. In the first instance, where feasible and once the new growth has hardened (is no longer soft) get the hose out and set it on a fine mist with high pressure and direct it at the infected part of the plant - you may need a ladder for this job. Be very thorough, as a small amount left behind can re-infect the tree again quickly. If you have further problems you can also try spraying the tree with Neem Oil.
Healthy trees are less prone to pest and diseases so regular feeding and attention is important to improve the plants vitality and fruit cropping. Apply Daltons Citrus & Fruit Fert every 6 weeks from October through to Christmas, recommencing again in February to mid-April.