Definitely read up on bacterial canker and see if that fits the symptoms. If bacterial canker seems like a possibility, be very careful about not touching your other plants or using the same tools after handling the affected plant, as it's easily transmissible through contact. The brown leaf margins with a yellow halo are a characteristic symptom of bacterial canker, but it certainly could be something else, so don't be too alarmed.
It could be over-fertilization, stress from temporarily drying out, or just general stress from high temperatures or who-knows-what.
Scorching of the older leaves can be due to potassium deficiency.
The other possibility is early blight. This isn't what it normally looks like, but sometimes it presents this way on some of my plants. If the plant is generally healthy except this symptom slowly works its way up the plant as the plant grows, it could be that (especially if you see normal early blight symptoms on any of your other plants).
So, my advice: if the symptoms match bacterial canker and the plant seems sick, pull it. Otherwise, wait it out, babysit the soil moisture, and see what happens.
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