ok, answered part of my own question here. It looks like it is a bacterial infection of Erwinia carotovora. Although this disease is correlated with plant cuttings.

http://www.plantfinder.com/Services/ARChase/yucca.asp

All of the plants that this has affected are very large long term mature plants.
There has been no transfer of anything (e.g. soil, cuttings, contaminated clippers for trimming, etc.). It has destroyed nearly 6 of my 8 large san plants (typical variegated speciies seen in stores and the shorter broad leaf gray type ). I'm very upset about it and want to know how i can further protect my remaining sans and the new ones which i will get to replace these.

thanks again

Hi Zach,

Erwinia is a 1st enemy of sans. collector and sans farmer here (Indonesia). You can loose thousand of your sans. in the same time!!! It attack especially for the 'thin' sans. ie: trifasciata, hahnii, etc.
SO, if your sans in serious condition, just destroy them and their media and quarantine the rest of them....
For preventive action, give your sans. good air circulation, put in direct sun and don't overwater....

Indra

The only way you can kill them is with too much water. Some love heavy water in the summer, others must be kept dryer. If the tip of the leaf is brown or white dried up and stiff it needs less water. It is is soft and you can bend it over, it needs more water and is a tropical species. Don't put plants back into dirty soil, or infested soil, it must be new soil and new pot. Or sterilized soil, and pot. These are the easiest plants to grow and need less care than most. I think Indra and I said more or less the same thing. She is very accurate. She knows what she is talking about only she is from a different country, that grows them so well, it makes me ashamed of mine. She has available rain water, which makes a big difference. Mine look more like native African plants. I grow mostly the species. Don't set them in direct sun, Don't use peat moss it holds too much water and for sure can rot them. I use a very fast draining soil, for most of mine, and they are doing just fine. Norma

Norma,

I'm male, LOL! (And 90% of my sans. group are male, I don't know why... ;))
Yes, I come from Indonesia, tropical country. We have lot of sun and rain ;). The best place in the world, I thought, LOL!!!. Why not? I bought Lav. 1970 from Thai and after 2 weeks, it has 2 pups ;)).
Norma, you have very very nice sans. collections including variegations and complete than mine ;))

Happy Gardening

Mr. Indra
The Weekend Gardener

Mr. Indra What a nice positive note, Thank you,
Most San. do very well in your country and make ours look sick. I grow mine very hard (min.water) so they look like African plants, I don't grow mine for size, I want them to look natural, so then I can ID them, I have most all of my species that are identified with documentation. I do not have all of the new species/hybrids which I am not sure if they are really new. I still look for Sans. that the Huntington Gardens don't have yet. Once in a while I get lucky.
I have found for instance that there are actually 2 varieties or forms of San. Bella, Len Newton. which to me looks like another S. suffruticosa.
S. humiflora, I don't know who to believe what it is supposed to look like. Some same it is related to another 'Baseball Bat' (S. halli) and others say it is a flat leaf, very wide and thick, but very short. Any idea's about this? I think someone is trying to create another species.
So I'm a collector and not on EBay. I think all of the 'experts' are splitting hairs to name new species. PS. I don't think San. get wilt, bugs, etc. they just get to much water in the wrong time of the year. They do go dormant between Oct. and April or June. Water the wrong time of year and they are dead, and it can happen over night. I let mine dry out between waterings.
Some of mine went down this year, and it seemed just the
S. trifasata sports, the cheap unimportant sport varieties. Norma

In this hot season (we have only 2 seasons here ;)) I try to water them everyday and fertilize them. But not in wet season.... I put all my sans. in open air without shading net.
PS:
I have no S. 'Bella', still rare here, (you're lucky cos you have many Botanical Garden;)).
S. humiflora, here we call it as S. 'Hawaiian star', I don't know who given this name...
I guessed humiflora as you said, has a flat leaf, very wide and very thick, then it called as 'The Giant Bat'.
Also as you said, there are many many invalid name for sans. here..... make us confused!!! Just want to raise the price??!!!

Happy Gardening
Indra

Hi Indra, I envy your growing condition.

Hi Z I'm going to try and respond to your question. I live in a very hot dry area, this year we have had high humidity so I'm keeping my plants dryer. However rodents can get into the pots and live in your plants pots they have the grocery store right above they heads, and plants for water from the roots. I have never seen San. get any kind of disease fungus, mealies, etc. only not enough water in the summer months because it doesn't hit the root ball due to the roots packed to tight to let the water get in. Perhaps if you haven't changed out their soil in the past 5 years it's about time. No matter where you live they will need a fast draining soil, and air circulation, they do not like stale air. Consider fertilizer when you get them re-established. Toss out old growth, use fresh soil, and new pots. Keep the heads with actively growing rhizomes puting several in the pots. Use about three young good looking new plants with roots per 8" pots, we only use 6" pots at the Huntington because of lack of space. We did not water during the winter months, other than just enough to keep them alive barely. Now we are really watering them heavy and they are growing like gang-busters. I'm not saving these suggestions will help you, but if they sound reasnable try them out. I don't see any one giving you suggestions, are you in the hot dry area of Utah? I think the ones you are getting from stores are the ones that like some humidity and lots of filtered shade. You don't seem to be growing the desert species. Norma

Indra, we are so dry here I have never seen it attack here. I don't think Utah has high humidity. I have a feeling that he is growing the tropical types with soft tips in a very dry location, he has given us very little information. We don't even know what kind of soil, pot, inside, or outside. We can't help him with out that information. I live in semi-desert contitions, very much like where they grow in Africa. Some grow along streams, some grow in old ancient coral reefs.
We need to know what kind he grows. Up northern utah they get snow, the roots may may been damaged from the cold, and it's showing up now. My advise then would be take the plant out of the pot inspect the roots, and take off all living parts and replant after they have dried out for several weeks and he sprays with Purex. I have a book that has four pages of step by step instructions, or I can copy off the pages and mail them to him, the book is out of copy right. So I waiting for information. No one else wants to respond except us, so it will be up to us to help him. These are common plants to replace, and not that expensive. I can't write a book here. Norma