Mold? Deficiency? Nute burn?

this clone is a 3+ weeks old and has been growing in soil under 42 watt light. i'm wondering what these ugly greyish-brown spots are. at first i thought it was mold but most pics of mold i've seen are white. is it mold or something else? only one of my clones is like this, and i feed the others the same nute mix. i need to find out what this is asap thx.

note: i've been feeding it very lightly with the N, but with 2 tbl of Fox Farm Big Bloom per gallon and some superthrive. thats it.

i don't see this on the newer growth but the problem seems to be spreading to other leaves.

i did some research that suggests it might be Phosphorous (P) deficiency, does anyone agree? this confuses me cause it's a 3 week old clone in veg, why would it need phosphorous in veg stage?? I thought (P) was needed only during bloom. suggestions please

It is definitely necrosis, but I can't say why other than a guess. You mentioned you have other rooted cuttings that are healthy with no problems. My best guess would be to say that this individual cutting is either not rooting, or has very weak roots. If this is the case, you are poisoning the plant with nutrients. The water and fertilizer is keeping it alive, but it's not thriving because it is unable to take in nutrients (lack of roots).

Try pulling the plant and checking the roots. If there are plenty, and they're white, repot, and water, no nutes. IF you use superthrive, stick to the drop a gallon regiment. I think you have an injured cutting that lacks roots, and is going necrotic because it's using it's own mass to survive. It will almost certainly still root and recover, but feeding is not a good idea at this point.

Feel free to pm me, I could elaborate, but don't feel up to arguing with know-it-alls who only want rep points or validation. Hopefully I've helped you and you're on your way to great success!

Peace brother,

Jason

It looks like it might be a ph problem as my last grow had marking like urs thought it was a deficiency so added some feed with trace elements didn't help then realised to check my ph once I sorted my girls took off as for needing phos in yes they need it throughout their life cycle Not as much in veg as in bloom n trace elements good luck with ur grow m8

good general reference:

n=green, big, stalks and leaves
p=roots and uptake
k=vitality and utilization of sugars during flowering.

If possible, check and post ph. Low or high ph will affect uptake of nutrients...

read previous post. best of luck!