Japanese Maple Damage
I have a well established (approx 30 year old) Japanese maple. I noticed that one major limb didn't have many leaves this season. I did some pruning of dead branches on that limb, before realizing that it was basically the entire limb that didn't leaf out. A couple of branches on that limb did have a few leaves, but they have begun to die off - and it is very early in this growing season. So, at this point, basically the entire major limb has no leaves. The limb has a long black mark on it - that I had assumed was some sort of natural sap dripping. On closer inspection, I noticed that the limb has lost some bark on the the backside of the black mark. Also, I noticed that many of the leaves on seemingly healthy limbs are a little droopy at their corners and edges. I have attached photos (a) of droopy leaves on a healthy limb; (b) of the black mark on the "dead limb;" (c) of missing bark on the dead limb and what might be a new black mark forming on a currently healthy limb. I also have photos of the whole tree available you can gauge species, but I can only send you three photos at a time. Question: Do you know what's happening to my tree? What can I do to stop / help it?
Japanese maples often get frost cracks for the freezing and thawing of their bark in winter. The trees can usually heal these, as on your (c) photo.
The black oozing that you thought was sap could be slime flux http://extension.umd.edu/hgic/slime-flux-and-wetwood-trees-and-shrubs
or simply fungi growing in oozing sap. Slime flux is not harmful to a tree. However, this is the branch (b) that has no leaves and is obviously dying. It needs to be removed.
Japanese maples are notorious for dying quickly or slowly for no discernible reason. Unless you have changed the environment in some way that stressed the tree (regrading, etc.) it will be very difficult if not impossible to say why it is losing some branches. This does not mean the entire tree will die.
If you want a closer inspection, you can have an arborist certified by the International Society of Arboriculture look at the tree. They are usually associated with tree service companies, which do not charge for the inspection, only if you choose to have work done. Here is a website to help you locate an arborist: www.treesaregood.org.