What type of tomato plant is it, heirloom or hybrid for example? Mine have been doing this since the heat came up about 6 wks ago. I've planted only heirlooms and wonder if it's virticillium or fusarium wilt. I've looked at oodles of pictures and I just can't tell. But if you grow hybrids, look for seed that is V or F resistant. That will be my approach next year.

I feel your pain - deeply. Gardening is a lot of work and between the shade requirements, keeping bugs and birds out and various diseases tomatoes can suffer......it can be depressing for sure.

I've worked very hard at maintaining good, healthy soil with lots of compost but grow mostly tomatoes and chili peppers which are of the same family and crop rotation for us backyard gardeners is a challenge.

Try a little fish emulsion

I did. No change. But dashygirl, try it and see. Maybe your issue is different. Let us know if it helps. I added lots of fresh worm castings too. Waahhhhh - sad sad tomato plants.

They are all heirloom - cherry, yellow pear and sweets. Should I use a soil test kit?? I've read that the acidity needs to be specific for tomatoes to thrive?

Just an FYI:
Somewhere, perhaps here, I read about planting sunflowers to shade tomato plants. Purely by accident, a giant sunflower came up at the perfect angle to shade one of the tomato plants from the hottest sun of the day. What a great idea! I plan to do this - deliberately - next year.

I had no idea giant sunflowers got so big. It is about 1.5 feet taller than my 6 foot block fence and the stem, or should I say trunk, is almost 2 inches in diameter near the bottom. No bloom yet.

I've had all kinds of weird problems with tomatoes until I started growing the hybrid big beef tomatoes. I have yet to see anything outright kill those plants. My plants had bad horn worms one year and they stripped the leaves off of 2 of the plants, and they both came back fine.

Just a small warning though: the plant gets huge and has a lot of tomatoes, so give them room.

I have shade cloth up covering these guys. So I don't think it's from the sun. And it started happening before I got into the triple digits.

I talked to someone at Arbico Organics in Tucson, one of my favorites places to shop for garden solutions, she said when it starts getting hot the plant will sacrifice the lower leaves in order to keep producing fruit at the higher level. It's a thought............

Looks like it is on one side of the plant = environmental. Sun, wind, heat.

There is a fellow who has a lot of videos on You Tube. He had one on tomatoes yellowing when it gets hot. He said some types get yellow and others do better. His Youtube name is rustedgarden.

On a few of my tomato plants (I planted 11) the lower leaves are yellowing, but all the rest of the leaves on the same plant are vibrant and huge. I even have one plant that appeared to be going all yellow, but as it continues to grow (and grow and grow!) the new leaves are normal and it's producing a good amount of fruit. So there may be nothing to fear. I foliar feed liquid seaweed, and would recommend it or fish emulsion as well.

My tomato plants looked like yours a few weeks ago. Just keep up with regular watering and light fertilizer and hopefully they'll perk back up. Mine are really enjoying the heat.