It is a young tree planted here spring of 2013 and was doing fine until sometime last week while we were on vacation and came back to find half the leaves black and dead. I am not sure if I can save it and if I should be concerned about the problem spreading.

I planted two contorted mulberry trees at the same time in a large raised bed filled with mushroom compost garden blend. I planted strawberries in the same bed as an understory. At the time we were free ranging our chickens and they took an affinity to dust bathing at the base of one of these trees and despite my best efforts to protect the planting hole they kept digging there so this tree became slightly tilted. I took care of the chicken problem and this spring and summer both of the mulberries have been growing like crazy. Very lush, green, and healthy. A few weeks ago I noticed that this tree had tilted over even more to a very crazy angle. It felt wiggly like the roots weren't very well anchored. So I put in two stakes and a bunch or string to hold it up straight (making sure the string was spaced out to not pinch the trunk) and put extra compost around it. I started giving it extra water about two times a week, just letting the hose run on it for a few minutes to get a good deep watering.

It was fine when we left last Saturday for vacation and we had someone water the garden for us while we were away. We came home yesterday and found half the leaves on this one dead. This morning I looked again and there are even more leaves starting to die just in the last 24 hours. If it continues this way there will be no leaves left in a week since it is still a young tree. The leaves turn black and curl starting from the tip, and the older leaves were the first to go. The new growth at the tips looks ok for now. It does not resemble fungal or bacterial spot, it is the whole leaf. The branches look ok and are not turning black. The other mulberry that I planted at the same time is doing fine, still lush and healthy.

Did I damage the roots by tilting the tree back up, even though it felt like it wasn't rooted in very well to begin with? Am I overwatering it? Is it sick? Do I need to be concerned about the condition spreading to the other mulberry?

Thank you for any advice!

Prune away the damaged area well below the damaged tissue and discard (do not compost) it to the trash. Stick your finger down into the soil (if you can) and make sure there is moisture there...but not wet.
Be prepared to lose that tree...you may get lucky and it may recover, but looking at the photo on a young tree like that I can tell you it doesn't look very promising.

Stay tuned for other responses. There are folks here who grow Mulberry and are familiar with it (I am not).

I googled bacterial leaf blight and you're right, the symptoms do seem to match. The USDA says mulberry is a known host and the disease clogs up a tree's xylem, restricting water flow. It makes sense then that the symptoms of the disease are similar to the symptoms for water uptake problems. I really hope it is "just" root problems and not that disease. It sounds terrifying! O.o I did not know that mulberries get problems with root rot so maybe I brought this on by accident.

Here is another pic showing the bed this is in and the other mulberry. (Hopefully this one comes through right side up....) My native soil is a dry riverbed. It has excellent drainage but is too rocky to dig so I built the raised beds on top of it with cinder blocks. This being the trees' second year and with them growing so well without interference, I was not watering them very often for most of the summer but kept an eye on them. I really only started watering this one more deeply when I staked it up, because I thought it would help the roots recover if I gave it some extra care. Since then and depending on how hot it is each day I've been watering it more. Every other day when it was in the 90's. Every 3-4 days when in the 70-80's. I try to go by how dry the soil is. I don't measure, though I'd say a few gallons each time. Enough to penetrate the soil without staying soggy. I have been watering the other one at the same time but not as many gallons.

I haven't applied any chemicals. The neighbor uses herbicide on their driveway but as far as I know not on their field. We have well water so there is no chlorine or anything. We've had two or three mole holes pop up but not near the garden and there has been no new activity when I stomp them down so it doesn't appear that they've colonized enough to be an issue.

The more I think about it the more I suspect that the combination of root damage and a sudden increase in watering are to blame. I would be very glad to hear what others have to say as well.