I think there are can be many causes for chlorosis such as your pictures show. My experience is that most often its a temporary condition and resolves itself in a month or two--or at least no more than an annual event. I think the key here is to not jump to any fast conclusions or make any hasty radical changes. But a little Epsom Salts is OK to do right now. If your soil is dry enough, make up a gallon or so of Epsom solution and water in. If its too wet, then do what Rick is doing. Then wait a week or so to see the results. I don't think your plant condition is all that unusual-- Casa Blancas grow so fast, sometimes they tend to get a little blotchie from time to time--mine do.
Whatever you do, don't monkey around with you soil pH abruptly; overreacting too quickly can lead to a ton of problems if not done properly or if done with 'I think it needs sulfur' approach-- without first haveing a years worth of symptoms, and second, haveing some numbers to back it up. Then, and only then, would I begin to make ammendments 'slowly' over the next two years.
Edit: In my walk around this morning I noticed my El Condor grew so much the last couple nights, it too looks just like your condition. I'm going to let this take care of itself. If you want to add Epsom salts, a quarter cup to a half cup per gallon depending on the size/area of the plant. Some may use more. Epsom Salts won't affect your pH significantly.
