This appears to be a fungal or bacterial issue. There are a few things that you can do. I have a recipe that I use that is very broad, powerful, and most of all... safe. If you don't have every ingredient, it is ok, but please don't skimp on soil priming. It is important.
The recipe is as follows: First, you will want to prime the ground with dolomitic lime, then spread wettable sulfur to Kill off any infection in the soil. I Also like to spray the plant itself. Mix a few drops of one or more of these ingredients into warm water with a spoonful of coconut oil: Rosemary oil, lavender oil, neem oil, garlic oil, cinnamon extract/oil and for really bad cases, also add clove oil.
For other helpful information please visit this link: https://www.gardeningknowhow.com/plant-problems/disease/root-rot-in-garden-plants.htm