It is a bacterial soft rot (Erwinia or something similar).
Cut the leaf off with a flame-sterilized blade, making the cut an inch or so below that brown patch. Allow the cut to dry completely before you water the plant again (2 or 3 days). When you resume watering, keep water off of the leaves.
I find that I have fewer disease issues if I provide plants with supplemental Ca and Mg. I do this with about a teaspoon each of Epsom salts and gypsum in a gallon of water, adding several ounces of that to a gallon of water each time I water the plants. Others may recommend other Ca and Mg supplements, those are fine too.
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