Could be erwinia, pseudomonas or pythium.
Get rid of the leaf, inspect the plant for other areas of infection.
Treat with a physan and copper fungicide for the first two; pythium can be treated with Subdue Maxx which most people don't have $$$ so you just have to
Sunburn would have a more distinct demarcation and not a pale yellow green area between the brown and green.
Repot and keep on the dry side.
Good luck!