Certified GKH Gardening Expert
If this is indoors, then with persistence and fungicide use and not re-using infected soils, it can be eradicated rather easily. This article will guide you on fungicide use: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
If this is outdoors, then you will have to use fungicides, in combination with constantly removing any infected growth from on and around the plant. You must keep all infected growth cut, and treat every year for the fungus. I recommend an annual treatment of DOLOMITIC LIME and WETTABLE SULFUR once or twice per year if you need a second dose. This will help condition the soil, and keep any pathogens at bay.
This article will offer some more information: https://www.gardeningknowhow.com/plant-problems/disease/root-rot-in-garden-plants.htm