Certified GKH Gardening Expert
If it is root rot related to drainage issues, you need to improve drainage. Often drainage issues are related to lack of drain holes in containers and clay or low spots if in the yard. If in a container, make sure the container has drainage holes and that those holes are open. In the ground, these articles will help you fix the possible issues:
If the root rot is caused by a fungal issue, in most cases, treating the soil with a fungicide will correct the problem. There are a few forms of root rot fungus though that cannot be killed with a fungicide and the plant must be removed and destroyed.