First, do you see any lesions on the leaves?  I don't think that your problem is scab, but just in case, look at the leaves.  Scab forms lesions that are less than 1/8 inch and circular and have a fuzzy greenish black color.  I think that you more likely have winter damage.  We have seen lots of commercial apples as well as crab die off because of cold winter injury.  Often these trees just appear sickly and ultimately die.  Lots of trees died earlier this spring, but many are still collapsing across the state.  I expect they will continue to do so over the next two years.  There are really no tell tale signs that convince you it is winter injury, so I can't give you a leaf symptom or tree symptoms.  I would suggest you fertilize the trees with the hope that they are just weak and will improve with fertilization but I am thinking they may not.  Sorry for the bad news.