United Kingdom 
I have a john downie crab apple tree which is coverefd by a fungus like cotton wool It has stopped the tree fruiting this year and appears to be killing off the tree. Can this be treated and if so, what with
31 Jul, 2010
Previous question
This sounds like woolly apple aphids which suck the sap and produce a woolly like wax. A severe infestation will damage the tree.
The natural predators are ladybirds so you can encourage natural predators, but as it's not a tree whose fruit you will eat, unless you are going to make crab apple jelly, you could try a systemic insecticide.
31 Jul, 2010