What is eating the leaves of the plum tree and making them fall off?

The leaves of a plum tree may be falling off due to several potential pests, such as aphids, spider mites, or caterpillars, which feed on the foliage. Fungal diseases like powdery mildew or leaf spot can also cause leaf drop. Additionally, environmental stressors such as drought, overwatering, or nutrient deficiencies can weaken the tree and lead to leaf loss. It's essential to inspect the tree for any visible pests or signs of disease to determine the exact cause.