Withering Trunk and Immenent Death

i have no idea why this is happening, but i've lost several plants now as a result. the trunk of the plant withers up right where it comes out of the growing medium and shortly after the plant, which is now being robbed of nutrients, dies. this never happened to me in dirt, but is an ongoing problem in hydro.

Perhaps pythium.

Sounds like a pathogen issue because your not using H2O2.