I’m not an experienced gardener; however, a gardener reminded me of an issue with watering that had not previously crossed my mind. First, not to water plants other than in the morning prior to it warming up or in the evening after the day has cooled off; because of the next poimy...also, when watering plants, make sure that either or both, Your hose water and/or your drip system or timed watering system water has had time to cool off or to run the water that was sitting inside your hose or water system‘s tubing, because water sitting in either a hose or tubing, can come out of the hose/tubing at over 100 degrees and literally cook your plant, when you believe you are watering/helping your plant..and during the day if you water your plants, any water left on the plant’s leaves will act as a magnifying glass and cook that leaf...so always run the water out of your hose/tubing until it has cooled off to less than 80 degrees (cooler is better obviously) before you water your plants and ALWAYS WATER the soil, not the plants...

Ok, so back to your specific topic, a quick google search lead me to what seems like the common answers (plural), with regard to yellowing tomato leaves, that include: Under Watering (obviously not your issue), and Blight in two fungus forms. Both, sadly, are stated as untreatable..and come with the direction to Destroy the Plants and rotate away from that soil as blight fungus will likely remain in the soil and affect next year’s crops If growing tomatoes..

Having said that, via the pics, I did notice that You have LOTS of leaves lower on Your tomato plants, which, as I understand it, is not a good thing, as leaves lower on the tomato plant make the Plant much more susceptible to developing issues, because leaves block constant air flow they need, and the more lower leaves there are, the more likely that when watering, the water will splash back up from the soil and hit the plant‘s leaves transferring fungus from the soil to the leaf, and causing blight, etc.

What I found on gardening know how dot com is the recommendation that You contact Your local Agriculture Extension Office, as they are most familiar with pests and disease in your area, and can provide you the best direction on how to tackle your plant and/or soil issue(s). Not sure this link will post, but it is to a page where you can search for your local Agricultural Extension Office‘s contact information:

https://www.gardeningknowhow.com/extension-searchI 

Personally, If you don’t want to go the route of ’destroy everything’, and depending upon what the extension office recommends, I would recommend (Trying with one plant): a) pruning back the first 4-5” of leaves off your plant at the bottom; b) Separate healthy plants from sick as far from each other as you can, c) Separate sick plants from sick plants spacing several feet apart at the least, d) I recommend this treatment test with at least one plant that you’re willing to take the chance of losing...Take some peppermint oil (organic), a misting spray bottle, add about 20 drops of peppermint oil into a 3/4th full of distilled or bottled spring water (pure spring water not the junk that has added minerals, is just tap water run thru a filter, etc. so read the labels — point is to use water without any chemicals or additives that are potentially in tap water or even mineralized bottled water), shake water/oil mixture, and using the mister, at night after it cools off, spray BOTH the Tomato Plant ALL OVER, and then spray the top of the soil..You are looking to ensure contact of the mixture with the plant leaves, stems AND the top of the soil, how much You ‘soak’ the soil is up to You and if You have worms in your pots or not..If worms, then try not to soak the soil (the peppermint will kill the worms, insects, and most nematos in the soil, good or bad) if no worms, feel free to soak away, as the hope is for the peppermint oil to kill off the fungus on the plant and in the soil....I spray my cabbage, tomatoes and other plants with this mixture once or more a week (more if I see eaten leaves, but I have worms in my soil so I attempt to just hit the plant and not to soak the soil, and my plants seem to love it, and perk up afterward.), and next year I would probably repot with a Mixture of good organic soil mixed with some worm castings and a teaspoon of Azomite for good measure, to help revitalize the soil hopefully minus any fungus issues...

Anyways, that’s my 2 cents, for what it’s worth...Good Luck to You & Your Plants, I hope things aren’t as dire as most sites seem to infer they may be..and that Your plants recover soon!...

thanks so much for the detailed response! I'll do my best to cut back and try your suggestions. It does look like fungus, which means a sad death for my poor plant :(

Yipes! You’re welcome, and Sorry to hear that, but Good Luck with the treatment, I really hope it works for You & Your Plants..and Best of Luck with Your Garden..*smiles*..Stay Safe & Healthy out there!..&..Good Luck with Growing Green...

Remove the brown leaves. They won't get better. Leave the healthy leaves. If you're still getting tomatoes, let the plant keep going. Sometimes they recover, especially if the fungus is weather related - moister and cooler than normal. If the stems look okay, your plant may surprise you.

Here's a but of hope for you ... mine got something early in the season and I kept cutting off dead/dying leaves until I had plants that looked like something out of Dr. Seuss. I truly despaired but I sprayed what leaves were left with Daconil. I watered with a very weak fertilizer solution. Lo and behold new growth started coming on and I have at least 2 tomatoes and all kinds of growth & blossoms. So take heart!