The answer to 'why is my sod disease' is that your new sod may be suffering from a fungal disease or pest infestation. Common causes include improper soil preparation, overwatering, poor drainage, or exposure to excessive heat and sunlight. Symptoms may include discoloration, wilting, or patches of dead or dying grass.
To troubleshoot the issue, inspect the sod closely for signs of fungus or insects. Adjust your watering and mowing schedule, and consider aerating the soil to improve drainage. If the problem persists, it's best to consult a professional landscaper who can properly diagnose and treat the issue.