The yuccas on my family's property have been there since at least 1970. It wasn't until recently, maybe the last 2 or 3 years, that they've begun to look like they're dying. Random stalks have begun to break off of the main trunk, random leaves have whitish grey coloring or begin to die altogether, new leaves in the center somehow become twisted at their tips so that instead of opening up and out they are pinched and tied together, a white/clear hard crystalline substance can be found near the bottom of the leaves where they attach to the stems (on every single yucca), I have seen some small white worms on the stems after I cut them off to propagate them... I'm at a loss. Most of these issues don't seem to fit any descriptions of pests/diseases that I can find online. Unfortunately I don't have any pictures currently to upload.
Certified GKH Gardening Expert
This sounds like a big problem. I will have to recommend a few things.
First, you should apply dolomitic lime to control the pH, as it sounds like it is slowly getting off kilter.
Second, You will have to find something to control the insect problem. There are many things that can do this organically. I use neem oil for many insect pests, personally.
These articles will help you: https://www.gardeningknowhow.com/special/organic/