My large magnolia trees, about 35 years old, are being afflicted by a black tarry sticky substance which causes the leaves to shrivel and fall. This works its way from lower branches to upper branches. Perennials growing under the trees are afflicted with this, but so far the leaves are hanging on. Online was a suggestion to spray with a solution of 1 Tbsp apple cider vinegar in one litre of warm water. I did this yesterday. Will this be effective at stopping it? What else can I do? I’d appreciate any help you can give me and thanks.