Hey there everyone, I've seen this in multiple different gardens and with entire populations. I'm pretty sure it is "salinity stress," and may be happening due to heavy well water, calcerious native soils, or salt bond partner buildup from salt nutrients. Salinity stress is confused for verticillium and other pervasive wilts in vegetable crop.
Basically, solution pressure in soil for these salts is so high, the plants must uptake some amount in order to drink at all. The lowest branch or first divergence from the xylem is kindof getting dumped on with salts, until they are so built up in the tissues that they're causing cell lysing and death, and the branch eventually wilts.
Consider this a guess.
Edit: check out this article:
https://www.marijuanatimes.org/sodium-is-on-the-podium/
"Some plants have evolved mechanisms to combat toxic build-ups of salinity by collection and extrusion, compartmentalizing excessive salts in vacuoles, or storing the salt in lower, older leaves."