Might be worth it to check the stem and around the base of the plant and look for disturbed soil. It's possible something was digging down (or digging up from below) and damaged the root ball.

My eggplant tend to drop some bottom leaves as they add new growth. Peppers and tomatoes do the same, and those are in the same family as eggplant, so it may be just what they do.

Other than that, water might be a factor. Or the high humidity might be letting some kind of fungal or mildew infection spread.

Well, took a look tonight and see patterns in the leaves I didn't see before. Upper leaves seem to be affected too. Bad feeling this is mosaic virus? Only other thing I can think of is per the root disturbance idea above that I just pulled garlic in the square foot rows next to the eggplants- about a week ago. Went to look for disturbance and found I created some of my own! Some were as close as 8" from the stem of the eggplant.

Could be mosaic. Any evidence for bumps or warts in the leaves? Mosaic doesn't leave smooth leaves. But it's true, eggplants lose lower leaves naturally.

Could be verticillium wilt.

See my thread: http://forums2.gardenweb.com/discussions/3134161/verticillium-wilt

i have had that too. i just cut them off. today my entire plant was cut in half. i was devastated.

Some of my lower leaves are turning under, and am guessing they'll soon start to yellow. The plants are getting fair water and great sun, but here in Ontario we've had a month of sustained heat and humidity and almost no rain. All the same, this happened last summer, and I cut off the lower leaves after they got yellow, and eggplants still grew! This summer, I've spotted 3 eggplants, without really looking too hard. They don't look like the leaves in the pics provided though!

It seems to be spread now throughout all the leaves-upper and lower- except the newest at this point. The plants are still growing in height though, and I would say about 1/3 of leaves are healthy looking- for now! Still no blossoms turning into eggplants. Sprayed with a copper fungicide, and keeping well watered so will see what happens I guess. It seems at least that the spread of folding leaves has slowed down quite a bit.

Well, what a difference a week made! Came back from a short vacation to find the new leaves all healthy and only a few older leaves with the splotchy yellow. Even those didn't fold up and look pretty strong. And I found a 1" eggplant growing! Looks like it outgrew whatever hit it before. Not sure if it was the extra watering, copper fungicide or fish/seaweed emulsion but happy to see I will have eggplants after all.

I vote for overwatering...not the heat, eggplants thrive in the heat.