The answer to 'why is my mulch disease' is that mulch can sometimes develop fungal growth or mold, which can make it appear diseased. This is often caused by excess moisture, poor air circulation, or using the wrong type of mulch.
Common symptoms of diseased mulch include discoloration, a musty odor, or the presence of visible fungus or mold. To prevent and treat this issue, make sure to use a high-quality mulch, spread it in a thin, even layer, and ensure good drainage and air flow around the mulch. If the problem persists, it's best to call a professional landscaper for assistance.