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?
“Late blight” possibly.
This is not a criticism , but they seem crowded, we all work with the space we have. Also, have you planted tomatoes in that spot before? That can cause issues too.
Don’t over water, let them dry out a bit, remove the dying plants asap, use a copper fungicide, and next year replace a lot of the soil if you have to plant in the same spot. If you can, rotate your crops every year.