The answer to 'why is my deck disease' is that your deck may be experiencing a fungal or bacterial infection, often caused by moisture, poor ventilation, or improper maintenance. Common symptoms include discoloration, rotting, or deterioration of the wood. To address this, you'll need to identify the specific cause and take appropriate action, which may involve cleaning, treating, or replacing affected areas.
If the damage is extensive or you're unsure of the root cause, it's best to consult a professional landscaper or deck specialist. They can assess the condition of your deck, diagnose the problem, and recommend the most effective solution, whether that's a DIY repair or a more comprehensive deck replacement.