If when your plants start to set blossoms, you may notice some yellowing of the bottom leaves. It's okay: your plants are sending all their growing power to the blossoms now. So don't worry. The black on the bottom might be back splash from watering or too much rain? Is that possible? I hope this helps. I am a newbie this year myself.
Good Luck,

Dusty

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It sounds like one of the "wilts" which is common in constant moisure, like excessive rainfall or over watering. It seems to be very common this year and is doing a job on my cantaloupes.
Try to keep the soil just moist and not wet, and they may recover on their own.

Ron
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The yellowing is on the oldest leaves which may be the problem -- they're old.

That said, the plants don't appear to be the rich green, nor as sturdy as they might be.

So several questions:
Are you growing in soil? If not, what?
Also, have you used any fertilizer? If so what and how/

This looks either like some sort of nutrient deficiency or salt intolerance or both - I don't see any bacterial or fungal signs. Hard to tell with the pix given, but is this bed new? What are the constituents of the soil? Too much fertilizer for salt? They look a little leggy too.

Dan

As Dusty and Jean indicated and as I have seen the older and lower leaves of tomato plants will often simply yellow and drop off the plant for many reason having nothing to do with any disease. The plant may need the nutrients from those leaves to support newer growth, the leaves may have been shaded too much and were not supplying enough nutrients to the plant to warrent being fed so that is cut off, many reasons. If the plant is growing well and otherwise looks really healthy and there are no black spots on the leaves I would not go looking for some disease.

So glad I read this thread! I noticed my otherwise healthy and vigorous tomato plants had lower leaves looking dry, curly, etc. Yes, they are in blossom, with a few small tomatoes on some. Now I know why. Should I remove those stems like I do the suckers to help increase fruit yield?

I expect the lower leaves on my toms to die off late-mid season and later. IME this is a little early. YMMV.

Dan

Yepper I remove my yellowing leaves at the bottom as I see them coming and it should help novice:)

Good Luck,

Dusty

Thank you all for your advice. I do think it is just that the plants need energy elsewhere - not a disease. It hasn't spread anywhere else. I pruned some of the yellowed leaves off and they seem to be doing better.

I am using the square foot gardening method because I have a limited amount of space in my yard where there is enough direct sunlight to grow veggies. I adapted a couple of things to make sure it is organic. So, my soil consists of 1/3 compost, 1/3 peat moss, and 1/3 course-grade vermiculite. So far, I've been very impressed with the results.

Again, I really appreciate all the help. Happy Gardening!