If you know your tree is sick - follow these steps:
Identify the disease. Are there options to correct the disease? Or will the plant have to be removed?
Determine why the disease occurred. What are the underlying health issues that led to this problem? Has something in the immediate environment changed that could have encouraged the disease?
Fix the underlying issues since this is probably why the disease occurred in the first place. Even if you have to remove the plant, this may save others from a similar fate. It may be a good idea to have the soil tested to make sure the disease was quarantined in the plant.
If it is possible to fix/suppress the disease, are there organic options you can use to do so? There are organic and non-organic compounds that will take care of diseases and pests. The system that we use is a closed system and focuses the chemical into the tree so there isn't overspray.
Continue practices that maintain and increase the health of your trees and shrubs, once the disease has been controlled or suppressed. Rake up diseased leaves or pick up infected limbs. If left to degrade these will reinfect the soil. Hydrate and fertilize your tree accordingly. Healthy trees are happy trees!
You can always contact a Certified Arborist with help of any of these steps.