no one can identify this??

First one of my 6 plants got it and had to kill it now second got it too
- soft wilting stems
- main stem has a big bulge just under the soil
- bottom leaves yellow
- no difference when dry or when watered
- stem just looks strange
- looks to be shrinking every day at week 3 now first one died a couple days ago

Humidity 39-45
Temp 28 celcius
600w hps
Organic slow releasing fertilizer and biotabs

Sounds like dampening off...have you had them too wet recently?

Yes how often are you watering? and, why do you have a pencil stuck in there?

Yes i think its dampening off too should i remove it from the other more mature plants i am watering every second day