Phal leaves suddenly turning yellow & spongy

[goulstone]

Thanks to all who posted; the picutures in the links were helpful -- I'm thinking its either erwinia or a possible thermal injury. I looked up the temperatures on the day my mom brought it to my house and found it had been ~10 F

Update:
Less than 36 hours after my original post, the leaf 2nd from the crown spontaneously dropped off the plant. In attached photo 'dropped_leaf' you can see the healthy tissue to the left and the diseased/damaged part closest to the monopodial stem.

Today, I cut the diseased/damaged parts of the crown leaf back as far as I dared and treated all open wounds with cinnamon. In the attached photo 'cut_leaf' you can see yellow had begun to turn black.

Is there any chance this was caused by exposure to cold? Guess I'll know in a few more days if the patient is going to survive.