Japanese plum trees are hardy and disease-resistant. They produce a juicy, plump fruit that may be green or yellow or even purple or black. However, these trees are susceptible to a few problems and diseases if they are not properly maintained, and failing to prune regularly and harvest appropriately can create difficulties for your Japanese plum tree that result in loss of fruit or even the entire tree.
Plum Leaf Blight
While European varieties of plum tree are more susceptible to leaf blight than the Japanese plum tree, leaf blight can impact any plum tree. At first, the leaves will show dead, gray, brown or black tissue in spots. Then the spots will fall away to create a hole in the leaf. You should remove affected leaves from the area completely--do not just drop them on the ground--and you can treat the tree with a fungicide if it has not already flowered.
As the name suggests, black knot causes black galls to form on the branches of the Japanese plum tree. These knots tend to appear on new growth in the springtime, so monitor your Japanese plum during the growing season. Black knot is a bacterial infection, so you can ward it off by fortifying your tree with nutrients, removing the affected areas or possibly with a specialized pesticide.
If you start to notice thick, black, powdery mold on your Japanese plum, then you may think that you have a powdery mildew problem. However, Japanese plums seldom get powdery mildew. It is more likely that your plum tree has been infested with scale insects that are producing a sweet substance that attracts ants and sooty mold. Look for hard bumps on the bottom of the leaves, yellow foliage and even lost leaves. If your plum tree does have a scale insect problem, you may have to remove the tree to save the rest of your yard, though pesticides may work if you catch the bugs early enough.