Fruitless mulberry disease

Question

The trunk of the tree is leaking sap causing a large dark stain

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Answer 1

I would look to be borers. These will destroy a tree, and is usually a terminal illness. You can hold them off for quite some time, but the tree will, eventually, die from this. This article will help: https://www.gardeningknowhow.com/ornamental/trees/tgen/tree-borer-insects.htm