Most of my tomato plants are doing really well. But the one in the cage highlighted in red started failing a few weeks ago. I fertilized with some Miracle grow I had lying around and it didn’t improve. Then I got some Tomato Tone. That did nothing. The plant kept getting worse and worse and now it’s just a husk. I’m afraid blue is following next and I’m concerned about purple. Here’s another angle for some more context. 
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Any thoughts?
At first I thought it was one of the “wilts”. These are soil-borne pathogens that attack the plants roots. The causitive organism could be verticillium, fusarium, or phytophera. In small plants pythium or rhyzoctonia can kill them. There is also bacterial wilt that causes the rapid decline of the plants.
Then I zoomed in and took a closer look at the plant. I suspect it’s nitrogen deficiency. It could be caused by over-watering (denitrification and leaching nitrate out of the soil profile). However I suspect you didn’t have enough to start with.
The tomato tone looks to be a 3-4-6 fertilizer. To put it simply, it’s a stupid fertilizer blend. Plants need nutrients with a ratio of around 3:1:2. So you need 3x+ more nitrogen in that blend.
Once the plant sets fruit, it starts to dedicate nitrogen into the fruit/seeds. In a shortage situation it moves them from the lower leaves (they turn yellow and die).