So, I pulled up the Morsel_B tomato plant, today. The whole plant had gradually started wilting (more like curling, and gradually dying in the process, I guess) last week some time, for no apparent reason (all the leaves at the same time), and it became apparent today that it probably wouldn't recover (as the stem was starting to look stressed, too). There was no chlorosis that I recall. The interesting thing, though, was that the soil and the roots smelled strongly of mold/fungus.

Have you heard of a fungal tomato disease that you can smell, before?

The roots looked fine (but they were pretty potent).

Maybe that was just microbial life in the soil decomposing stuff, though. I don't know. The soil probably wasn't high in organic matter, but I did add fertilizer to it once or twice (fertilizer can stimulate microbes).

Before it started wilting, I hadn't pruned the plant (so, it wasn't pruning stress). and also, before it started wilting, it was remarkably vigorous. The fruit was still tiny, though (but I don't know how big it was supposed to get).

I had a plant that suffered similar symptoms last year (Horse_A), but I let Horse_A decline further before I pulled it up (I don't recall any smell last year, though). I'm pretty sure it's not physiological leaf curl, as it affects all the leaves at once, it continues in severity steadily, and the new growth doesn't look promising.

I'm guessing the smell was just a response to the fertilizer. I'm still stumped about the cause of the plant's decline, though, unless it's bacterial wilt (which kind of matches the symptoms). Maybe I should fish it out of the garbage and do another test (I tested a branch of it earlier, but I think you're supposed to test the trunk).

I have a feeling it's just Verticillium wilt, without the chlorotic blotches, because I found two other plants (a wonderberry and the Broy_A0 tomato) that were starting to get the same symptoms, but they also had Verticillium-style chlorosis (which I'm already familiar with--I'm less familiar with the plants actually dying because of it, though). I pulled those two plants up.

Anyway, it feels like I'm thinning the garden to give the other plants more room.

I think it’s true that you can smell certain tomato diseases.
I know when I’m nearing the start of the trouble period by how the plants smell.
As to what’s generating it … early blight and septoria are the 2 things I’m always hit with, but I’d have no easy way to sort out which one is contributing an odor. Maybe both are.

Now I want to get a microscope to have a look at them. Has anyone here ever actually attempted that with their tomatoes? Apparently you can with at least some diseases (Verticillium included, if you know how). I'd love to see pictures of any microbes you've found in your garden.

@PlainJane
That reminds me of how our tomato plants smell like BBQ sauce sometimes, when they start to get old and the weather gets colder. I wonder what causes that smell. I never noticed it until the year I grew Palestinian and smelled it on it (I think that was 2018), but in years following I smelled it on a lot more plants.

The soil for Morsel_B smelled like that spicy/peppery potato dirt smell (I mean, sometimes potatoes smell like that--not all the time); I've smelled the same smell in potting soil, too (which at the time had a lot of something like fungus gnats in it).

I think I can smell pithium, or maybe something else that thrives in the same conditions. It usually happens when the roots are starting to decay. No foliage disease though.

Tomato plants are so smelly themselves, I'm at a loss to have ever noticed the smell of a disease... But the smell of the jungle in my greenhouse at season end is overwhelming if I let it get to that stage. Leaf smells, resins, whatever it is.
Molds and rots just seem to be visually gross, to me.
I suspect I'm missing the fine points, since the tomato plant smell itself is not pleasant to me.

I like how tomato leaves smell. I keep meaning to make a fresh tomato leaf salad or a chopped fresh tomato leaf topping in lieu of basil for a caprese salad, but haven’t done it yet.

Never smelled any tomato fungal diseases. Some fungi do have atrocious odors. Stinkhorns do. A mildew infested enclosed space has a wet, unpleasant odor.