I don't have any ideas but I'm having the same problems this year and it's really bad. Out of 9 plants 2 are on death's doorstep ready to be pulled and 2 more are getting symptoms so whatever it is it might be spreading. I suspect blight. We had tons of rain last month (over 10 inches). When my plants had those yellowing leaves like yours I pruned them off. This was 3 weeks ago. The pretty much dead plants just kept going downhill. I was going to search the tomato forum to figure out what I did wrong and maybe try not to do the same thing next year. BTW: Due to all the rain here in Chicago, I have mushrooms growing out the drainage holes in many containers.

For your tomato to die that quickly it probably isn't something you did, as long as you treated it well. I have grown tomatoes here for many years, and they are surprisingly difficult here because of fusarium wilts, and nematodes. I mostly grow cherries tomatoes now because of the fruit fly problem. Tomatoes are very sensitve to a fungus at the spot they emerge from the soil. Slugs will eat around the stem and kill the tomato quickly also. We have a lot of slugs in Hawaii.

Sorry, no picture has appeared.

The brown you are seeing is just the inside of the tomato that has been exposed to air and darkened. The tomato skin split and splitting is quite common in early spring because of the inconsistent soil moisture levels. It often begins with what is called "zippering" (Google tomato zippering) and the zipper then splits open. Just like the zipper on our jeans can if we wear them 2 sizes too small. :-) With tomatoes it is a flush of water intake that causes it to split. You can find many pics of the various forms splitting and zippering take in the discussions about it over on the growing Tomatoes forum here. Dave