Q: My very old mimosa tree has black gum spots on the trunk. 

A: It is a sad fact that mimosa trees, despite their beauty, are very susceptible to a specific disease: mimosa wilt. The fungus lives in the soil. It enters the tree through the roots and travels upward. Over a couple of year’s time, limbs turn yellow, wilt and die. Eventually, the whole tree is dead.

Late in the progress of the disease, the trunk will exude gum. That’s what you’re seeing on your trunk. There is nothing you can do about the disease. It only affects
mimosas, so you can plant any other tree you find attractive in that spot.