Heat treatment is widely used in nursery practice as a way to sterilize living plant tissue of persistent viruses.  This technique is typically employed with tissue culture propagation, where a very small apical meristem 'micro-cutting' is subjected to high temperatures for a given period of time.   This kills the viruses without killing the plant, kind of like an immune system fever response.  The plant tissue is then grown out into a 'clean' plant.

There have also been anecdotal growers' reports on this forum about fmv(fig mosaic virus(es))-infected plants getting cured by high and low temperature extremes.   I'm quite curious to hear any other reports by those who have had that experience.

As I grow out my crop of fig starts, I'm seeing the signs of fmv on 60-75% of the plants.   Some of the 'clean' plants are one of a multiple set of the same variety in which others display fmv...others I've received the same variety from multiple sources, and one source's plants show fmv and the other does not.  Most of the in-ground 'Unks' I've received have grown out clean foliage.   Is fmv truly as ubiquitous as some folks claim?

For my eventual in-ground trials, I need to grow out the healthiest plants possible, so it seems important to address this issue.   Perhaps for others in warmer climes, fmv does not impact the plants, but for zone 5, I feel I need every advantage.   

Although I don't have the laboratory setting to do tissue culture, I have started a heat treatment experiment with some of my duplicate starts most heavily afflicted by fmv.  A little bit more of a hill-billy approach, but using some scientific methods; careful monitoring with pyrometer, control group.    These plants are being placed next to a consistent heat source, my woodstove, for a bit of tough love: 24-36 hours of high 135F + temps.

The first one I did this with defoliated, but now 3 days later is starting some regrowth which seems healthy thus far!   

I don't want to open a can of worms with this discussion, so no opinions, conjecture, please.  Previous discussions on fmv seem to sometimes get a little off track, and I would like to just stick to the topic.  Thanks!

Yeah for an extended perioud of time you can kill viruses and bacteria. I do cook with this method using water baths, thermometers, and plastic bags. Sous vide I think its called Where you cook it at low temperatures in an extended time to kill bad bacteria and viruses. What remains is very very tender and juicy meat. ;)

Ascpete, who is away for the winter, is doing the exact same trials. Pete S. It would be useful for both of you to compare notes and share with the forum, he won't be back til spring I think.

I believe this is the thread you are mentioning:

http://figs4funforum.websitetoolbox.com/post/fmv-free-uc-davic-black-ischia-and-uc-davis-black-madeira-7040439?pid=1285258128#post1285258128

Seems like it was never updated with results, unfortunate.

Bass also mentioned this when he interviewed the Russian fig researcher. http://figs4funforum.websitetoolbox.com/post/show_single_post?pid=41259085&postcount=11

I would suspect that the best results would come from holding at high temps for several weeks and then tissue culturing a bud.

The idea behind thermotherapy is to denature the virus (cease replication) with heat (~95f) while the plant is still actively growing. The new growth now contains no FMV and is removed and propagated for virus free plants. That is my understanding anyway. 

Higher temperatures for shorter periods may not have the same result, I don't think the virus is actually destroyed and could begin to replicate again when the temperature is lowered. I have never tried either method, so keep that in mind. 

From my personal experience the combination of FMV and the fig mite is much more damaging than either alone. Infected plants grown without the fig mite can show fewer symptoms as time goes on, some cuttings from these infected plants will not show any symptoms themselves and may be virus free. The virus itself travels through the plant slowly from cell to cell, while the mite is extremely mobile by comparison and will spread symptoms to the entire plant. I think that is why freezing to the ground has a positive affect, it spurs rapid growth that the virus cannot keep pace with.

There are several other viruses that may or may not cause any symptoms together or alone, FMV is scientifically proven to cause symptoms. Not every tree has FMV, that is a fact supported by many studies. Fig mite symptoms can easily be mistaken for mosaic, and they do spread FMV so can cause every tree in a collection to be infected eventually. At the same mite symptoms may be masked by more severe FMV symptoms. If you would like to read more about fig viruses I compiled some data from different studies here.

That was a very cogent explanation, thank you hoosierbanana.

Hi Zone5figger,
In pots, fig trees can suffer lack of some nutrients and that leads to misshaped leaves and the grower thinking about FMV.
I don't say that you don't have FMV, but just that everything you call FMV might not be FMV, but problems in the growing conditions of the trees.
Try fertilizing more one tree in your test group.
70% of plants with FMV seems a lot to me .
As already mentioned watch out for bugs as they can produce same bad looking leaves as FMV.

I think I need to see validated virus testing from an laboratory before I did the happy dance.
Virus have a nasty habit of going dormant for a while and then making a comeback.
I agree that many things look like virus that are not.  In orchids we see a fair amount of fungus that makes leaves looked virus.

Here's examples of the fmv on my starts-      
All these are receiving identical treatment to healthy plants like this one:
 

Pic 2 looks like mites: pale spots and browning.

The others look like FMV: vein clearing, mottling, deformity, stunted growth. 

Thank you all for these posts. They are very enlightening.