Looking into tomato diseases I found that most of the fungal infections have yellowing of the tissues surrounding the brown/gray/darkened spots. My tomatoes don't have any yellowing until the whole branch has withered and is falling off.
Since they were clean seed, planted in clean soil-less mix, in clean pots, and were inside the whole time I've kind of ruled out any soil or pest-borne diseases.
But! I did find out something I didn't know, probably because I've never started my own tomato seeds before and had them survive this long. Purpling of the leaves is an indicator of phosphorus deficiency. This makes sense, they have been in those little pots full of seedling mix forever, and only had fish emulsion for nutrition (high nitrogen, barely any phosphorus or potassium). So if they are purple because of lack of phosphorus, maybe their are deficient in other things too... it turns out that potassium deficiency causes browning of the leaf margins and curling of the leaves. In fact, many nutritional deficiencies cause some type of discoloration and leaf curling.
|Sweet Million 4 days ago|
|Sweet Million today|
One of the things that tipped me off was that the Sweet Million was transplanted just a few days ago and is now looking much healthier.
It's difficult to tell in the picture, but the earlier shot had a purplish stem and leaf color to it. It's very obvious that it's a bright, healthy green now; though the leaves that had already curled are no better, the rest look great.