Certified GKH Gardening Expert
This sounds, either, bacterial, or fungal in nature. I have a remedy that I use personally to treat the ground, and the plant. It is harmless to the plant, and much less toxic than conventional methods. It 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.