Do you have a heavy set of tomatoes (fruit)? Have you been fertilizing well each week?

If It's only on the lower leaves, I would tend to lean towards the plant being undernourished. Tomatoes will routinely cannibalize their lower limbs to keep proper nutrients to the plant for fruit production. If you see the yellowing and dieback of the leaves continue and spread after you increase your fertilizer, then you can eliminate that possibility.

with your spraying regimen I think that it's unlikely for it to be any kind of blight or fungus.

I do have a lot of fruit setting in...I have attached a picture and I think you may have a good idea with being malnoursihed. I have dark landscape cloth around the base of the plants which lead to it being close to impossible to water them efficiently. I purchased a drip irrigation system to get the water near the base, but I have not fertilized because of theis issue. I have a fertilizer injecter coming that I can hook up to the system but do you think I may be too late?

Nope, you're never too late, just mix up some miracle grow, or some organic garden tone, or any kind of liquid fertilizer in a watering can and feed those little guys. I know what you mean about the plastic, I can't stand the way that the water runs off. This year I mulched my tomatoes with hay to even out the moisture content of the soils, and so far it has worked well. For fertilizer I used the jobes fertilizer spikes and they have worked exceptionally well. You can find them most anywhere.

Just try to get some nutrients into the soil under that plastic. You'd have to ask someone else about how well drip systems work, I haven't tried it yet, and I don't know how well the fertilizer attachment would work. If you're worried about disease, after you fertilize you can pinch off and remove all the discolored leaves.

Regarding the tomato fruit, from my experience, the types of tomatoes that I grow usually turn the green-white just before they ripen to red. Your fruit looks healthy so I'm guessing you'll just have some tomatoes ready to eat soon.

Your tomatoes look really nice!

I am also gardening in zone 5 and I start the season with the black mulch in order to warm the soil as much as possible.

Some time in July or August I remove the black plastic, add some granular slow release fertilizer and a thin top dressing of compost. Then I mulch with a heavy layer of straw, like 6-8 inches.

With the plastic mulch it is tough to water beyond the base of the plant. You want the tomato roots to go broad and deep, so watering all around the plant in a 1-2 foot radius is ideal, IMO. Hence the permeable straw mulch. It's just faster and easier to get the watering done, especially in this dry summer.

Watering with plastic mulch also tends to expose some of the roots at the base of the plant which I make sure I cover with the compost. I also use a liquid fish/seaweed emulsion spray as a foliar feed every couple weeks.

Good luck!