How many plants do you have in that bed, and can you tell us what varieties they are?

I'm not sure what it is, but if they were mine, would definitely be spraying with a fungicide just in case. My fungicide of choice is Daconil.

BTW, very nice plants.

Thank you for replying.

I have 4 plants, each in a 1' square. I have this beefsteak, then I have 2 medium size slicing tomatoes (I don't know what kind) and then a cherry tomato.

I'm trying to stay organic, is Daconil organic?

I have 4 plants, each in a 1' square...I haven't added any type of fertilizer.

Then when they are planted that intensively, that over-crowded - what is their source of nutrients? What is feeding them?

You are attempting Sq. Foot Gardening, correct? The plants still have to be fed and fed regularly. Especially after periods of heavy rain that dilute and wash out any nutrients in the soil

What soil mix, media are you using? Is this a new bed? If so then it has no active soil micro-herd, no bacteria to provide nutrients to the plants and you will have to supply them. There are any number of organic fertilizers and fungicides available but no Daconil is not organic

I don't see any signs of any disease issue in your photo but I do see plants that have been over-watered or over-rained and in need of feeding.

Dave

Thank you. I bought some tomato-tone and kelp meal for them. The mix is 1/3 compost, 1/3 vermiculite, 1/3 peat moss.
I don't like that they are crowded so much. Next time I will only put 3 in this space. Live and learn.

Daconil is not organic. Now might be a good time to research a good organic fungicide. Those are nice plants and it would be nice to keep them that way.

If you are ORGANIC, there is NEEM OIL .
I have both Daconil and Neem oils and use them both.

Yellowing and eventually aborting leaves is the plants natural response to its limited resources. Lower leaves get the pink slip first to reduce the burden on the plant.
A well fed young plant seldom will abort leaves.