what disease does my plum tree have?

Question

Its leaves are turning red to yellow and falling off. It is a Santa Rosa plum tree.

Answer 1

It may be a nitrogen deficiency. A severe nitrogen deficiency can make the leaves red and yellow and prone to fall off. If you are also seeing spots, it may have bacterial spot.

Try giving the tree some fertilizer and if you suspect bacterial spot, this article will be helpful:
https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-diseases.htm