Which disease do you think it has?

I'd echo Moon grower's question above - you can't treat a disease without first knowing what it is.

We would need an exact description, and pictures would be invaluable, particularly of the parts that are in the process of going down. Cultural information--sun exposure, watering during drought, soil type and slope, nearby structures and trees, etc.--would also be helpful in getting a sensible answer, Tbrien.