Root rot and collar rot can be caused by a number of things including:-
* planting too deeply in the soil or
* heavy rain and humidity in the warmer months.
As a preventative, always plant high in pots and in the ground no deeper than the plant was when it was purchased .
Keep mulch away from the stem or trunk of your tree creating a mulch ring.
Mangozeb is an effective system fungicide used to treat Rot.