Thanks very much, here are some thoughts on your input:

Heat stress: Nope, it happens at the bottom first.

Nute burn: Nope, I have backed off a little on nutes and some leaves are even turning a little pale and tips are not burned like that.

Humidity: Usually my plants have loved the low 20's and 30's humidity in flower, don't think these would be any different.

Root problem: Well, maybe I suppose, I don't have much experience there. I transplanted them not too long ago and the roots looked fine and well developed, just gave them some more room.

Overwatering: Well, it certainly is pretty warm/hot in there and I know that adds to the potential lack of oxygen to the roots which can cause root rot. I have seen that before but it really didn't look exactly like this, then again, maybe it doesn't have to, but it is odd that the upper part of the plant is not affected (yet). I do use a bit of hydrogen peroxide when watering to try to get more oxygen down there but I think you could be at least partially right here, though normally I have seen the top of the plant drooping too when overwatering is the issue.

Anyone have other thoughts? Can we really completely write off all types of mold?

Thanks again....
Bulster Reviewed by Bulster on . Is this Mold? What to do? PICS..... Hi all, I'm in very early flower indoors under 1000w HPS, in soil and coco fiber w perilite. temps are 80's and 90's. I feed with 10-30-20 one part nutes. Something is affecting my plants, it starts on the grow tips of the lower branches and works its way up the plant. Affected new growth tips are droopy and turn black or grey and die while still very small, essentially ending most growth of the branch it heads. Also, some leaf stems suddenly wither and shrink to the width of a human hair, Rating: 5