Thank you for contacting us about your plum tree. This appears to be cytospora canker, a common fungal disease of stone fruit trees (peach, plum, cherry, etc.) and many other tree species. This fungal canker affects the plant’s vascular system above the canker (similar to a wound or “sore”) by blocking the nutrients and water flowing up the tree (thus causing the leaf issues you are seeing above the canker). Cytospora primarily attacks trees under stress. I am including a link to some great information on cytospora; it will answer the questions you will have on causes, treatment and care going forward.

Our peach growers on the Western slope battle this disease and is a significant cause of tree loss for them.

Cytospora canker: Cytospora Canker (colostate.edu)

Thanks again,

Karin