Hello, Last spring (2012), I thought this one was a goner, like others that didn't survive the weird winter we had in 2011-12, but I wanted to save it. I removed the dead bark until I found some "green" (cambium) with a clean sharp knife, applied copper sulphate (Bordeaux mix) diluted with water to make a kind of paint-like paste, applied it to the wound, let it dry for a couple of days, then put some wound-sealing paste I use for my bonsai. And apparently it worked. After one season, the tree has developped a nice, healthy scar. New buds have even appeared on growth rings that stayed dormant for years. Now, I'm planning to "refresh" the scar when it's active (April-May). See the photo with a lght green line. Another solution would be to carve and treat the dead wood, and then make a "bridge-thread" graft. Healing would be even faster, but it would require the same variety as the rootstock to be "elegant", otherwise it might bulge, or on the contrary be weak, or lead the way to other problems.
I'm not sure I would want to mess with it if it's doing well. In time as the tree grows, don't you think the wound is small enough that it will completely fill in? If you do decide to graft some bark on there, please follow up with pictures and let us know if it's succesful, I'm very interested to see how it goes. Good luck.
I'd cover over the wound as seen in the middle photo with the tree sealant. I've been a proponent of using tree sealants for Maples and other trees and frankly do not care what anyone else feels about the use of such a strategic and useful tool for bark repair and subsequent wound restoration. It is textbook or it used to be for us working with tree wounds to use a bordeaux paste prior to us using a tree sealant as a cover. Everything you've done should help this tree for the short term. The near term is that there is no real remedy for this tree due to the amount of bacteria in the rootstock itself. As long as you keep this tree growing in a vigorous state, you should okay but when this tree stops or slows down in its rate of setting out new vegetative buds there is a strong likelihood that this tree will start its final period of decline that it will not recover from. I would not graft over this rootstock since the disease issue can be seen in the rootstock itself. The graft is not to my liking but it was not the graft that is your problem as there is a healing where the scionwood and the rootstock wood have co-joined together. The issue is the rootstock from just under the graft that has a bacterial disease that left unheeded will in time be the main cause for the demise of this tree. In other words any time you have a bark split in the understock below the graft due to a pathogen, this is the precursor for the fungal alboatrum in the plant to in time, no matter if the tree stresses or not, lead to the overall collapse of the tree usually in a three year decline for us here. Sometimes as long as a five year decline in a cooler region than ours. Also, in your attempt to help this tree along I suggest you either pot this tree in a much deeper container or better yet plant it in the ground. Keeping the roots so confined, while working on the bark wound, to such a small area can lead to a stress that this tree does not need at this time. The more vigor you can impart into this tree or allow the tree to generate on its own, the better chance you can help this tree for the long term. Jim
I didn't work on the scar as I had planned to, but the wound is healing. Here's a pic taken today (sorry for the poor quality):
