Certified GKH Gardening Expert
This appears to be a fungal or bacterial issue. I use a treatment that is well suited to cure either type of infection. If you don't have, or can't get these ingredients, you do not have to have them all. It is just more broad range if you do have them. The recipe is as follows:
First, you will want to prime the soil 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.