The answer to 'why is my mulch disease' is that mulch can sometimes develop fungal diseases or mold growth. Common causes include poor drainage, excessive moisture, and using low-quality or contaminated mulch. Symptoms may include discolored or slimy mulch, musty odors, and the presence of fungal growth.
To address mulch disease, start by improving drainage in the affected area and allowing the mulch to dry out. You can also try removing and replacing the affected mulch. In some cases, a fungicide may be needed to treat the underlying issue. If the problem persists or spreads, it's best to consult a professional landscaper who can diagnose and treat the issue properly.