Viruses tend to affect new growth, not the leaves near the bottom of the plant. Sadly, though you've been spraying so vigilantly with daconil, I think it may be falling prey to a fungus anyway... daconil doesn't combat systemic fungi. If the plant takes something up from the soil into the roots, there's very little that can be done.

My best guess is either Fusarium or Verticillium, which are both soil-dwelling wilts. To test for them, you can cut into the stem a little at the soil line, deep enough to see the fibrous "wood" core. Either one will turn the wood brown, instead of its normal creamy white. (Little strips taken off the stem at the soil line with a sharp knife don't seem to hurt plants, at least if there's only a couple of wounds, so you aren't going to finish off the YP if you do the test.)

You can leave the yellow leaves on or take them off, as it makes no difference to the spread of the disease. The good news is that if it's Fusarium, some plants get hit with it and then recover again, so it might survive (Yellow Pear is close to being a weed anyway). Verticillium is a little less sunny in its outlook. Both persist in soil for seven years or more.

Don't plant tomatoes, eggplant, peppers, or potatoes in that bed again, and try not to spread its soil to the rest of the garden on tools or shoes (raised beds make it easier -- I track mud everywhere in my flat garden). If you have a persistent problem with it, the only solution I know of is to plant in containers raised off the ground to prevent soil contamination... or take your chances on whether the plants will be able to pull through the infection.

It's a difficult disease to deal with, but there are people who manage. Good luck.

--Alison