I found this website and never thought you could air layer such a large trunk but it shows a technique that might help you but you must keep it in a pot and not have contact with nematode contaminated soil.  Air Layering  This individual has a nice website showing some unique varieties.

Thank you very much!
This is very interesting to see what other people to do!
I may try this method on couple of my trees..
I all the time try experimenting and most time I get lucky with solutions..
I guess as lon as you looking .. you will find the answers, or cure..
Thank you very much again!

Hi Olga,

Hi, Cecil!
Thank you! I trying this one now on Kathellen Black..
This my first air layer..So, I'm kind worry..

I find I have nematodes on two of my oldest trees.
I planted my Jelly Fig and Green Ishia along with 3-4 other cultivars about 10 years ago.
I ignored JF&E's advice to enrich a large planting hole with at least one half mixture of pine bark or other mulch along with mushroom compost and cow manure along with half of the original soil (sand). First year lost all except the Jelly and GI. May have been the todes or my watering.
It is hard to describe how fast this North Fla sand dries out and actually repells water once dry. I think it would be hard to overwater in the summer in pure sand here.
The GI and Jelly survived,grew huge and after a couple of years produced big crops of figs. Coulda been the rabbit manure.
Then two years ago theystarted to weaken and decline.
So.... I don't know if the weakning of the trees gave the todes a foot hold, or the nematodes finally took over.
My later plantings were done with a big hole filled with pine bark,mushroom compost/cow manure, dolimitic limestone and soil, with a heavy mulch covering it all.  So far so good. Fingers crossed.
Still trying to save the two big trees with heavy mulch and compost and TLC. 
I want to try grafting onto some LSU Purple rootstock and have started several airlayers.
If all else fails I'll just airlayer the trees and go to large pots.
FWIW the ivermectrin (for livestock)MUCH Diluted, really does kill worms including heartworms in dogs. 
Wouldn't be hard to try some diluted on a potted plant that has nematodes to see if it really works.
I would advise great caution before eating figs from the treated tree though.
My best
Dave.

Hi, Dave!
My pots was cover with pine barks and before this I put in pots lots of lime and fertilizer.. I find out about nematodes when I start removing pine barks from the pots..
Nematodes like compost very much..
i remove all compost from pots..
If I will be on your place..I will try to put beneficial nematodes couple times.. And after this plant marigold or garlic under the trees..
I don't try ivermectin:-)
This was just idea..
About over watering.. Few years from now I kill couple figs with over watering here in Florida, so this is possible..
Also I think may be not a bad idea to cover all ground around trees with black plastic at list till end of summer..I don't think this will hurt your trees..
My friend have a Golden Celeste in ground..He put lots of mulch around the tree.. Tree very healthy and produce lots of figs every year..
I was hope myself on cow manurve.. After clear all roots from compost I put them in mix cow manurve, plus perlite, plus pit moss..
They don't die from nematodes, but they don't grow much, they just sitting..
But after I re pot this plants with Jangle Growth soil, they took off..
So, if I will plant any in ground..i will dig big hole and fill it with Jangle Growth soil and put lots of mulch after all..
And put lime and fertilazer 2 times in year..
Jangle Growth have pine bark in it..
I'm little bit worry about using pine bark in big amounts for a long time..
I think in the end this will kill the plant..
But in small amount I think this is Ok to use them..
Nematodes love sand, love pompost, so this is hard here in Florida to have inground trees...
I will try may be next year..
I want to wait..
Some varieties don't get sick with nematodes and all..
But some like Atreano, Mary Lane seedles will get it for sure..
So, for inground planting we may be will need to plant one what don't get sick with nematodes..

Hello Dave,

I don't know if you have a readily available inexpensive source of compost. If you don't, you can haul away free compost from the Apalachee recycling center not to far away from where you live. It is a little "undercooked" but any organic material would benefit the sandy soil you describe and did I mention that it is free?. You might even consider planting trees in pure compost initially.

Also, I don't have LSU rootstock but I do have a huge seed grown vigorous Celeste whose cuttings I plan using as rootstock for experimental grafts next year. You can have as many cuttings as you like. If you rooted them soon, you will have plenty for for next year. Let me know if you are interestred.

Frank

Olga, I thought that one of the ways to combat nematodes was to use lots and lots of compost. In theory, compost should help provide a wide variety of beneficial bacteria, worms, fungi etc that would "crowd out" (suppress) nematodes. I may be wrong.

Thanks for the tip on the compost Frank.

I agree . Compost and rich soil are recommended for nematode problems. I think that is one of the problems in our sandy soil,there is no organic material. Plus the soil in many areas were worn out by farming/pesticides/herbicides et al.
It may look like virgin forest,but 50 or so years it was all farm land.
Thanks again
Dave.

Hi, Frank!
All my plants was in compost with perlite when they got nematodes..
And I find some info.. Nematodes love sand and love compost..They don't like cow manurve and pit moss and pine barks..
Only one thing about cow manurve.. Figs seems don't real like it too, in my experience slow them down..
So, I still will continue to plant in profecional soil Jangle Growth from Lowe's, yes, this is kind expensive almost 12$ for a bag, but fig love this one and nematodes not..

Just another thought on nematodes/compost/figs.
Some compost like 'mushroom compost' are alkaline as opposed to the normal acid compost,that needs limestone added (for figs).
Does the alkaline mushroom compost discourage nematodes because they don't like alkaline soil or does it just make the fig roots and plant happier and stronger and able to fight off the nematodes ?.
Are LSU Purple really nematode resistant , OR are they HEAT resistant (they seem to be) and therefore stronger and again,able to withstand nematode attacks ?
I picked up the bottem bag of pine bark mulch that had been stacked under a shade tree. I noted that roots had grown from the ground up into the pine bark. Could nematodes now have entered the pine bark and into my potting mix along with the roots?
Short of sterile lab conditions I think it's difficult to come to real answers as to how the todes get into our plants.
But I'm thankfull for all the ideas, as they all make us think.
Best to all.
Dave.