Ganoderma is a common decay / root rot fungus. Your arborist is right - not much you can do about it. The decay is fairly slow, though, and trees may last for a fair few years after being infected. It enters trees through bark wounds, e.g. those caused by lawnmowers colliding with trees. I'd be planting some new trees now, so that they are well established and a good size before all of the hackberries have to come down. And make sure they are well protected against bark damage! Resin

I really appreciate all the help you have provided. I see now that it is definitely not a hackberry tree. But if it is not too off topic to continue this I would be interested to know if it is a hickory, etc. SO here is a new picture, the leaf on the left is a black walnut, the one in the middle is the same as the picture above, the photo on the right is another mystery tree. Here is what I have always thought were elms ... the one on the left was a slippery elm and the one on the right was an American elm ??? The reason I am asking about hackberries is that since I started putting out fruit I now have lots of hackberries and I am curious if the host plant is something in my yard so I've been looking closer at some of the trees that I've never bothered to identify. Thanks again for all the help!

I don't think there is a Latin name beyond Candidatus spp.. It's a phytoplasma caused growth aberration that is many axilary breaks from a single growth/leaf axil. Witches Broom is fairly common in plant pathology literature, most often as a symptom of phytoplasmas (that used to be called MLOs, Micoplasma Like Organisms. Asters Yellows is a common one.) The link below is a good intro to what some of the phytoplasma caused symptoms look like. Here is a link that might be useful: Good intro to phytoplasmas

Sounds like you have everything pegged pretty well, except I wouldn't classify C.occidentalis as "almost entirely xeric". Although it occurs more frequently in upland areas, it is adaptable to a wide range of conditions including occasional flooding. It's unlikely that the possible C.tenuifolia was laevigata. They vary considerable in multiple factors including leaf type, bark, and habitat. You could have either tenuifolia or occidentalis. Two differentiating factors between laevigata and occidentalis are leaf margins and fruit. C.tenuifolia's leaf margin is nearly entire on the basal third of the leaf and it's fruit is orange-red when fresh and smooth when dry. C.occidentalis' leaf margins are toothed all the way to the leaf's base and it's fruit is dark purple when fresh and wrinkled when dry. I will attach a link below to the eFloras key to the Celtis genus. I just LOVE eFloras! Maybe one day it will be complete.