Definitely the first two pictures are looking quite unhappy. ID'ing problems like this isn't what I'm best at (other people are likely to show up who know more), but to start with the container looks very small for a plant that size. Once they are that big, they are hard to transplant. If you can move it in to a container with at least 5 gallons of soil, or more, that would be a good thing.
If not you are going to have to treat it like a hydroponics project, where all the nutrients are coming from what you feed it, not from the soil. It will need frequent watering and frequent fertilizing with a good organic fertilizer with lots of trace minerals and micro nutrients and not too much nitrogen (like Tomato Tone).
I'm not even for sure if that is a blight / disease or just a nutrient deficiency. I am leaning towards the latter, as there doesn't seem to be a lot of spotting, leaf curl, wilting, or other disease symptoms. Tell us more about what the soil is and what the conditions are (how much sun, how you are watering/feeding). Is it mainly the bottom leaves that are yellowing or do the new leaves at the top have it too?