Hi Mark-
It could very well be some kind of tomato disease- since Dats and Brugs, tomatoes, peppers & eggplants are all in the same family, solanaceae. So they are all susceptible to the same diseases. Sounds like a fungus or bacterial wilt. Copper fungicides help prevent the disease, and may help recover plants that aren't too far gone. Get a good vegetable spray with copper fungicide in it, and set up a regular spraying program, (weekly) to keep it in check. Hope it helps!
kasha77

Might it be caused by voles attacking/eating the root systems? Mark, are your datura potted or in the ground?

I think Kasha's suggestion sounds like the problem as it's only members of the Solanaceae family that are affected.

They're in the ground . I do believe that it is either fungal or viral, but am unable to determine which. It is so far clearly only solanaceae, which wouldn't be terrible if it weren't practically a monoculture in this bed. I've never had to apply fungicides before ; would a copper spray be reactive or preventative? All visibly affected plants have been torn from their homes, sadly, but would it be wise to apply to the entire bed? Is neem oil an effective enough fungicide?
Thanks a ton

Neem won't help a fungal problem. I don't know if the soil should be treated or if just removing all infected plant material is sufficient. You might want to contact an agricultural extension at one of the universities.