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Possibly a deficiency in Sulfur or Calcium.I suggest applying gypsum (calcium sulfate) and a full fruit-tree or palm fertilizer, high in micro-nutrients.
If you put the branches back together before the wound dried out, you can wrap it with plastic for a couple of months, to retain moisture and promote healing, and add some strap or duct tap over the plastic. If the plastic won't be shaded well as is, cover it with paper or a leafy pruned branch.
Carve the two wound surfaces, then quickly bind them back together. You can leave a couple of millimeters opening at the very bottom of the wrapping to drain excess moisture.