A couple of my Pim Seng Mun seedlings have this happening to some of their leaf stems. It's not superficial like sooty mold, but is in the tissue. What is it and what can I do about it? Thanks!
I believe that is a fungus. I have seen that in a lot of trees in SoCal, looks like you guys have it in Florida too. Sometimes you can see a hole where the darkening begins. It usually starts on that little stem where the leaf begins, just like the picture you posted.
In our colder climate, the darkening progressively moves down the central vein of that leaf and the leaf eventually dries up, sometimes staying attached to the branch after it’s dead. Sometimes, dieback of the stem occurs around areas that show these symptoms.
I would spray with Copper fungicide recommended for mango and alternating it with Sulfur per manufacturers instructions. If you use a systemic copper like Magnabon CS2005, it will work better than some of the cheaper coppers.
Simon