Certified GKH Gardening Expert
Unfortunately, your photo did not come through. I am unable to see the damage.
Ultimately, too much water may have allowed the fungus to enter the tree. In order to kill the fungus (if it is a curable one) will be to take away the conditions in which it will thrive. Since these conditions are quite the opposite of what the tree prefers, it will be quite easy to restore the environmental balance, unless rain is constant in your area.
Make sure that the soil has plenty of time to dry, thoroughly, down to about 3 or 4 inches between watering, or just let nature take over. Most fungi cannot survive very well in dry soil. The tree can, however.
A fungicide will, also, be necessary. There are many, and the best one can depend on the exact disease in question. This article will offer some information:
This article will offer information on the care of the tree: