While a bit over-simplified, it is fair to say that there are only three types of diseases that might strike the plants in your home landscape. There are fungal diseases, bacterial diseases, and viral diseases.
In the majority of cases, there is absolutely nothing you can do about a viral disease or a bacterial disease once it is established. The plant will die, and often in pretty short order.
It is the fungal diseases we deal with here.
What is very important to understand is FUNGAL DISEASES CANNOT BE CURED! You cannot bring back that part of the plant that has turned brown or yellow or lost its leaves. FUNGAL DISEASE CAN BE STOPPED FROM SPREADING ANY FURTHER AND IT CAN BE PREVENTED NEXT YEAR IF YOU KNOW WHEN IT IS LOOMING.
Fungicides are not a cure. They are a tool to stop the spread of a fungal disease that has occured or they are a tool to use to prevent an expected fungal disease from getting started.
As you can see at the left, you have a choice between "natural fungicides" and "synthetic fungicides" depending on your personal preferences. We need to point out that at this point the natural fungicides only deal with ornamental plants and some food crops. They do not deal yet with fungal disease in the lawn.
For more information about this problem, see our file in Yardener's Helper Dealing With Fungal Disease.