What is destroying my bean stems? Fusarium? And what to do about it?

A large bed of beans is stunted and sad looking, as always; I never seem to be able to grow a crop of beans.

I dug up a few and found that the stems have been seriously eroded, either by a pathogen or some type of insect pest. I'm thinking it might be fusarium root rot?

If it is, what should I do about it?

That to me appears to be either damping off or damage from some chewing nibbler. Damping off usually occurs with seedlings, and if brought about by wet conditions at the soil. Are those new bean sprouts or established bushes/vines?

Here's more on damping off: https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/cankers/damping-off.aspx

To me that looks like something is nibbling them, I would try transplanting some beans instead, let them get a couple of true leaves before planting them out.

The beans are about three weeks old. The damage is underground.

Not obvious in the picture, but the stem is sort of hollowed out.

I dug around looking for wire worms or other underground pests, and didn't find anything. I dug up more beans; they all have similar damage.