7 months ago
Last edited at 8:59AM on 5/25/2013
If you want a healthy tree and healthy fruit (my favorite variety of apple is the Honeycrisp), then you treat it immediately. Apple scab does not kill the tree itself but wounds it and leaves it open for other things to do so. Here is a link that explains how it harms the tree and how to treat it (scroll towards the bottom to "pests").
It is very important o treat Honeycrisp apple tree for scab because once fungus starts showing on fruits and leaves, the tree cannot be saved. However, you should treat the tree with an appropriate fungicide in advance. Planting resistant tree varieties is the best preventive measure.