Looks like severe nutrient deficiency with all the interveinal chlorosis and some powdery mildew. Container plants require regular weekly nutrient replacements to remain healthy since the nutrients wash out of the pot every time you feed them. And once the plants are weakened and the temperature and humidity conditions are just right Powdery Mildew will take over the plant. Use fungicides to treat it but you have to catch it early.

Dave

Agree on severe nutrient deficiency as a definite suspect. I would also want to rule out mites, specifically russet mites, especially given picture #2 and your description of dead bottom leaves as "dusty brown". Look at the undersides of a bunch of leaves that look like those in the second picture. Use a magnifying glass, as mites are often too small to be seen with the naked eye. Or you can put a piece of paper under the leaves, tap the leaves, and examine the debris that falls down onto the paper with a magnifying glass. Note that some mites can be seen with the naked eye if you have very good eyesight or with a normal magnifying glass, but if it's russet mites, they will be very small and may require a 14x or greater power magnifying lens.

Could also be the more common spider mite. They don't always leave webs. If the damage shown on this photo looks familiar, definitely check for some kind of mite.

I did fertilize several times over the season with Vigoro tomato plant food. It seemed excessive to do it more (this is my 5th year container gardening, and I have not fertilized more in the past). I know that my summer squash definitely got powdery mildew, but I did not see the same type of thrush on the tomato leaves. Does it present differently in tomatoes? Also I have one healthy plant growing next to it that is the only survivor! @gorbelly, that pic definitely looks familiar....

I really just hope it is not something that is soil-born and impossible to get rid of. I spent about 500$ on dirt for my beds and certainly cannot shell that out again or pay for removal! Yikes!

"I did fertilize several times over the season with Vigoro tomato plant food. It seemed excessive to do it more (this is my 5th year container gardening, and I have not fertilized more in the past)"

Nothing really wrong with that fertilizer even though it is high in nitrogen and a dry granular so not readily available to the plants to use like liquid supplements are. That availability can make a big difference when growing in containers.

But either way when the plants show clear signs of nutrient deficiency they need feeding. Growing conditions and demands vary widely from year to year. For example, greater heat equals more frequent watering needs and that equals more nutrient loss from the container.

And consistent availability of nutrients also plays a role. It does little good for the plant to have plenty one week and then little to none for the next 3-4 weeks. That is why so many container growers recommend feeding weekly with a 1/4-1/2 strength diluted mix of a well-balanced liquid fertilizer.

Dave

Pictures 2 and 3 are definitely different problems. I'm not familiar with #2 though it looks like a fungal blight, but #3 is certainly a deficiency problem. Whenever you see the veins OK and the leaf between weak, suspect a mineral deficiency. I would guess iron or zinc deficiency, which is not uncommon when you fertilize only with the Big 3 "N-P-K" commercial stuff. Try adding seaweed and compost. Those have a lot of the micro-nutrients plants need.

Iron deficiency shows up on newer foliage, not on older foliage, and it generally looks very different. I would suspect Magnesium or Potassium deficiency before any of the more exotic issues.

This page has some good information on nutrient deficiencies.

Sap-sucking pests can also cause or contribute to interveinal chlorosis.