Mesquite lower small tree limb leaves have turned brown as if lack of water. Tree has good water. Hard sap noduals a appear on these limbs only.
Certified GKH Gardening Expert
This seems to be common, and somewhat normal unless it spreads upward. Then I would look to a fungal infection. Otherwise, sometimes the plant sheds its lower growth naturally.
Alternatively, if you are feeding the plant, it could be fertilizer burn. These are actually legumes, and produce their own nitrogen. They don't, usually, need extra feeding.
Here is an article for more information on the tree:https://www.gardeningknowhow.com/ornamental/trees/mesquite/growing-mesquite-trees.htm