The answer to 'mulch disease' is that mulch can be susceptible to various fungal diseases that can damage your plants and landscape. Common causes include excessive moisture, poor air circulation, and using contaminated or old mulch. Symptoms may include discoloration, wilting, or rotting of the mulch and surrounding plants.
To address mulch disease, start by ensuring proper drainage and air flow around the mulch. Replace any visibly diseased or moldy mulch, and avoid over-watering. In some cases, you may need to treat the mulch with a fungicide. However, for severe or recurring issues, it's best to consult a professional landscaper who can properly diagnose and treat the problem.