The mites are either just that, mites, or else they're some form of bark beetle - these things don't kill trees, but they aid its decline, and usually only go for dead or dying trees. I'm interested in whether the orange deposit can be rubbed off at all - it looks like rust more than anything else. I thought at first maybe it was fireblight, but now I've seen the pics, it doesn't look like it. It may be that your old trees are getting ready to depart - you could try spraying with a fungicide to see if that clears the rusty deposits, but the tree is probably old ailing anyway.