On behalf of Dr. Glen Hartman: It certainly could be stem canker. The issue might be whether it is coming from the ground or not.
Most of the time with stem canker, it can occur anywhere on the stem but often towards the base of the stem just a few cm above the ground. Can you tell from these images if it is above ground or not? Also, it would be best to have the stems split to see what discoloration looks like in the stem. Other things it could be include charcoal rot, Sclerotium stem rot (not Sclerotinia) or possible some insect borer (split stems to see) or possible termites? Do you or Yechalew have any ideas? If note, this may be a good candidate for isolation if Yechalew thinks it is especially widespread. It seems very severe since it is killing the plants.