Hey Bro.....That looks to be fusarium/stem rot , and the cause probably came from those rooting plugs staying wetter than the rockwool cubes between feeds and never drying sufficiently , thus the girdling effect.....and hey.....
I assure you they`ll look perfectly healthy , but outta nowhere they`ll also fall over dead in a day , had a whole patch of tomatoes die in a couple days time one yr when it was raining every day so beware …..especially since it`s all 60 plantlets.....now....Could be your flood level`s too high when they`re fed and that`s why the stems are girdling from sitting in feed juice too long every feed , so.....things to ponder....and.....
Issack over in the grow diaries section had the same problem but with just a few plants and someone told him to spray a weak bleach solution and he saved his plants by doing just that , so go check out his 8K Coco Grow thread and you`ll find out about that solution to know how much to dilute the bleach....but.....
Finding the cause is as important as trying to save the crop so good luck and.....
Peace......DHF.....
G `day DHF
You called it .
Hot roots will cause it .
Next sign is yellow leaves that don`t fall off . And a damp basement smell .
Dunno if air layering will help the plant is infected . And pretty soon the room , the clothes you wear and anything within reach of the spores is a vector waiting for the conditions to suit another outbreak .
Gotta keep the nutes and roots cool or face crop failure .
To OP I would scrap that crop now and adjust climate conditions and start again . The flowers from those plants wont be marketable .
Thanks for sharin
EB .