Question

Whole tree  

Bark peel - 1   I have a 10-15 yr old ornament crab apple tree. It has been thriving and healthy for years. This year its bark starts to peel and expose bare wood beneath. When I touch the exposed wood, I can scrap some soft powdery substance that could be just the soft tissues.

I am seeking information about what is happening to the tree and what I can do to restore its health.

Answer
It looks like sunscald. Sunscald is form of winter injury that can cause cracks and splits. Sunscald occurs when cells in the living tissue beneath the bark thaw out on sunny days. This occurs mainly on the south or west side of trunks and branches. These cells rupture when they re-freeze at night. The tree is injured when enough cells in a given area rupture. You'll notice the injury the following spring as a discolored, sunken area. Fungus infections often invade trees via sunscald injuries. Young, thin-barked trees are most susceptible to sunscald injury. These include maple, honey locust, linden, and mountain ash. Heavy pruning on neglected trees exposes sections of bark that have been protected from the sun's direct rays for years, predisposing them to sunscald injury.

You can reduce or eliminate sunscald injury on young trees by wrapping the trunks each fall with tree wrap paper. Do this every year until the bark begins to roughen. This may take only a few years on some trees, but more years on others. Prune trees that haven't been pruned for years in stages, not all at once. This will help prevent sunscald.

I would cut the loose bark off making sure you do not cut into the tight bark. This will help the tree as it tries to grow callus cells over the wound.

I would spray the area with an insecticide called Merit to prevent borers from entering the trunk. There is a new systemic insecticide call Bayer Advance Tree ans Shrub Insect control that is mixed with water and poured around the tree and the trees roots take it up and if an insect bores into the tree the insect is killed. I would not use this IF you plan to eat the fruit but since it is an ornamental crab apple you more than likely will not. And fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. This will increase the health of the tree and allow the tree to better stave off any insect or fungi.
In general I would not be too concerned about the crack except for borer prevention.