Q.Why Is The Trunk Of My Bottle Palm Hollow
I have a bottle palm that is hollow in the lower portion of the trunk. The tree is 5 yrs old. I first noticed this when I found a weak area in the bark. I thought maybe this was because my sprinkler system was wetting the side of the trunk I adjusted it so it wouldn’t. I later pushed on the weakened trunk area and found that the area behind it was hollow, about the size of a basket ball. I have taken and enclosed some pictures of the tree’s exterior and interior as best I could . I thought at one point that the problem may be caused by ganoderma root & butt rot, but the tree doesn’t show all the symptoms. The bottom of the cavity appears to have some nesting material on it. I have sealed the opening with duct tape.

Certified GKH Gardening Expert
I definitely see signs of Ganoderma. Generally, once you are able to detect the disease, it is too late for treatment. I can see some of the white mycelial mass on the inside of the tree, likely, left over from rotting the crown of the tree. This type of damage is very common with the disease.
It is very difficult to prevent the disease, but usually healthy trees will not be infected very easily. Usually overwatering will exacerbate the issue.
Make sure that the ground is dry down to at least 3 inches, completely, before watering again. This can help to prevent future infections.
You can treat the soil around with a biofungicide, but it will not cure the issue. It may help to prevent the other palms in the area from suffering the same fate, though.
Here are some articles that will help:
https://www.gardeningknowhow.com/ornamental/trees/palms-trees/ganoderma-in-palms.htm
https://www.gardeningknowhow.com/special/organic/biofungicide-information.htm
https://www.gardeningknowhow.com/ornamental/trees/bottle-palm/caring-for-bottle-palm-trees.htm