This appears to be a fungal issue paired with a pH issue. I do have a method to curing this that I like to use. The soil priming part is most important as this will, both, correct pH and kill the infection. Although the spray is not completely necessary, it will help, and prevent insect problems in the future. 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.