Some deciduous euonymus plants will experience a natural shedding of some foliage, especially older leaves toward the center of the plant. These varieties will begin to shed leaves in the fall or the winter and go dormant for a few months. This is a natural process your plant undertakes as it prepares to sprout fresh new leaves in the spring. Cut back on watering in the winter to avoid root rot. You can use a liquid fertilizer in the spring to help give it an extra boost once it starts to put out new growth.
Scale Insect Infestation
The scale insect is small, oval and flat, with a protective tan or brown shell-like covering. Scale generally targets the undersides of leaves, around leaf joints, and along stems. They feed on the plant and often secrete sticky residue called honeydew. Quarantine the affected plant and rinse it off with cool water to knock off the majority of the pests. Remove the adult scale insects by scraping them off with a q-tip dipped in rubbing alcohol. Treat the plant with a commercial insecticidal soap or neem oil. Always follow the directions on the bottle. You may need to treat it several times to fully get rid of the scale.
If euonymus leaves are shriveling up and looking burned before dropping off, then they might be getting scorched from the sun. This can happen during hot summer spells, especially if the plant is not receiving enough water. If this happens, try to move the plant out of any direct sunlight or water with a bit more frequency. The euonymus plant does not need to dry out in between waterings completely, so keeping the soil moist, not soggy, might help solve this problem.