Can you provide some pictures of what you have. It should not die after being established. My guess is that you may have drainage issues or something wrong with the soil that prevents the roots from getting oxygen.

Some pictures of the culms and root balls would help.

This picture shows what I guess is a white fungus, on most culms. Next picture is a random shot of some roots,then i took a picture of some we cut out to show what i guess is more fungus. The patch hasn't been thinned in years but I'm guessing we are cutting out about 70%. Even last years growth has a lot of dead ones. Some have leaves half way up and top is dead. I do see a lot of shoots ready to start popping up. There is no standing water in the patch.The dirt is heavy and a lot of clay in it. The last picture show a part we haven't cut yet. You can see the dead ones Thanks for your help Jane The back side of the patch holds a little water when it rains, not in the patch but next to it

I think you may have an issue with your soil becoming too compacted, and heavy clay soils can be prone to drainage issues. Moso tends to produce relatively small delicate roots on huge culms so when it gets to a certain size, it becomes tougher and tougher to support. It will be easier to see if you dig out a section of the soil and look at what is going on. Most likely, the roots that are being produced are failing to support the huge culms.

I would suggest clearing out all the dead and dying culms so you can open up the canopy a bit. You could possibly amend the soil with lots of gypsum, but I think an even better idea would be to plant nitrogen fixer such as alfalfa or clover at the base of the grove in order to improve your soil. If you can get a few truckloads of mulch, that should improve the entire soil profile by promoting more bacterial and myccorhizae.

An even more radical idea is to raise some chickens and fence them within the moso grove to improve the fertility of the soil.

I thank you very much for your advise. Sending a few more pics. We have cut so many out, looking much better. Maybe this is a silly question but I still have so many small ones coming up. After all these years It seems they should all be big. Will try to add a lot of mulch and fertilize. Maybe the underplanting will be possible for me. Thanks again for your help and patience with my inexperience Jane

That is beautiful bamboo, I wish mine were that big. I do know that my bamboo love pee, it's some of the best nitrogen fertilizer you can get and it's free! I actually have some small moso growing, it had been looking pretty sickly and yellowish, and then we used pee and it started greening up quickly.

Thanks, Never heard of that one! Jane

yes urine is great fertilizer. Mix it with 10 - 20 parts water.

Anyway I'd cut out all the dead ones. Then get a truck load of compost, spread it out, keep it watered well, and let the new shoots and growth take over.

Although looking at your photos it looks very very wet and looks like a lot of rotting.

I agree. I used to have the same issue with poor drainage in my tree peony bed, but after I added a few bags of gypsum, it solved the problem and there was no longer any standing water.

Your moso grove may require a truckload of gypsum to loosen up all that clay. It definitely works because after the application, I no longer saw puddles of water and my plants stopped dying.

Looks to me like site is too damp also.