I'm not one to panic and I'm not there yet, but I need some helpful thoughts from the forum please to try to figure this out.  I'm afraid by the time I figure this out on my own I may lose a quarter of my trees.  And honestly, maybe that's what is supposed to happen.  This is my first time growing fig trees and growing them in Houston.  My goal was to see what does well here and what doesn't.  I still hate to lose one tree though.  Maybe that to is a fact of fig growing, eventually you will lose a couple trees just like starting cuttings.  My theory is, I've come this far, I don't want to lose any!

About 1/4 to 1/3 my fig trees have yellow leaves about 1/4 to 1/3 the way up the tree from the bottom.  My trees are in 3 gallon nursery pots and full sun all day.  My potting mix is 70/30/40 black humus / coir / sifted coarse perlite.  For my doubles I used MG moisture control potting soil.  The weather here has been near or at 100F all week and before that high 90's for the past month at least.  The pots are on concrete, but get some shade from one another.  I'll try to post some photos soon. 

Here's a couple of examples.  I have two Lattarollas.  One has no problem.  The other has yellow bottom leaves and also shows curled leaves and wilting when I get home from work.  These observations seem contradictory.....yellow for over watering but curled/wilted leaves from lack of water.

I also have two Sunfire trees.  One no problem and the other has yellow bottom leaves and 2-3 leaves on top also drying out.  Once again contradictory observations......yellow leaves from overwatering and drying leaves on the top of the tree indicating not enough water.

I pulled three trees from their pots and they are slighty to moderately root bound.  One strange observation to point out, one pot showed a moist (dark) layer on top of the root ball and a moist (dark) layer on the bottom of the root ball sandwiching a dry (lighter color) layer in the middle of the root ball.

What's going on???

Could it be the ones that have pots that are slightly shaded doing better than the ones that get full afternoon sun directly on the pot?  It could be that some of the pots are drying out too much or getting too hot in the center of the root ball.  That could explain the wilting.  I think over a period of time of drying too much could cause yellowing too, due to stress.

If that's the case, try shading the pots with a board and see if it helps.

I had some 2 gal pots on my concrete driveway in late spring and the heat from the concrete was cooking the roots.  The concrete has that light brownish pebbly layer on top.

Try putting them somewhere that they get morning sun and late afternoon shade.  The only tree I had trouble with dropping leaves is a BT in full sun all day.  During that hot spell we had last month when it hit 107 it stressed and a bunch of lower leaves turned yellow.  You are right that they will turn yellow and drop leaves when over watered too.  But given the heat, and the fact you don't have them sitting in a pond, I'd lean towards it being too hot for them on the concrete in full sun. 

It is not uncommon here in Houston for that to happen and the tree's to loose more than half of their leaves during the summer.  Just drive to the park in my neighborhood and you will see that.  There are several LARGE fig tree's in there that are almost bare.  But they come back to life year after year and put out an amazing crop of figs. 

Another thought, is it one particular potting mix that seems to be struggling?  Is it the MG on the ones that are suffering or is it the other mix?  It might give you an idea if one isn't draining properly or not holding enough moisture.

I'm sure you've thought of this, but is there any correlation to which ones are having problems and which soil mix they are in? Ex) are the primaries having trouble or the doubles or both with no consistency?

Edit) Ha, that's what I get for getting caught up in TV while I write a reply, Meghan beat me to it.

Wilting sounds like insufficient water. You may be watering enough but the soil is not absorbing it properly or sufficiently, or it drains too well - particularly if one soil mix is having problems and the other is not. You might need to pot the problem ones on a tub of water and let them soak for an hour or two to get them thoroughly wet.

Consider some shade, in the mean time to help them stabilize.

i would suggest poking holes into the soil mix and watering well, or use one of those saucer for the containers and fill that up with water and see if the tree soak it up. do a root work once the temp drops. first yr i was growing the fig, the root grew so fast, it filled out the container. i used plate under the container to have tree soak up the water. then it started rainning heavy and i didn't want to let them sit on water all the time, so now i use 12" screw driver and poke holes into the soil so the water can get to all the roots.

   I had a similar problem back in early July...about 15 trees started with the yellowing leaves (even dropping some leaves)...I immediatly transfered them to a shady spot on the side of the house for about two weeks...during that time I observed that the leaves that had grown during the early part of the season formed a large canopy over the top of each tree which caused the water to run off the leaves and over the edge of the pot and onto the groung (with very little water actually getting into the soil)...of course the inground trees  did not have this problem, because all the water that was deflected off the leaves hit the groung and went to the roots anyway...so now I am more careful to aim the water directly onto the soil of the potted fig trees to avoid this problem...by the way, even though some of the trees dropped almost all thair leaves, they eventually grew new ones, and all the trees survived and are doing well. 

Use a surfactant. Potting mixes come with a surfactant already mixed in but it washed away. Some people use soap, I use an organic fertilizer product that contains yucca, a natural surfactant.

I has stressed trees and it was the soil getting too hot in the black pots (smaller pots more succeptable)
painted the pots white and covered with burlap and issues went away

Soil in pot can get very hot in sunny day, even the weather is 80F. I have several pots with mulch only, I pull out the mulch to up pot some plants, and the temp inside must be over 120 F.