Possible P deficiency? please help

Ok so I have 4 plants on a 12/12fs light cycle and my Barneys Farm blue cheese isn't looking too hot. While the white widow and Big Bomb at the same age has started to show sex the blue cheese hasnt and instead is showing dark spots

Light 400w hps
nutes GH flora 3 part
Temps- a bit high around 30°c or 80°f

If you look at the last pic it almost looks like a burn except that it didnt start at the tip and also i havent even started using full strength nutes so i assume it could be deficiency.
Also if you look at this chart the pics match up the closest to medium stage phosphorus deficiency https://www.rollitup.org/attachments/marijuana-plant-problems/1894318d1317184266-guide-nutrient-deficiency-toxicity-deficiency.jpg

Heres what im talking about, Thanks in advance

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Ok i made a list of possible problems in order from most likely to least likely please let me know if you think im right or way off track

1. Nutrient Deficiancy-

Possible because-
The most I have fed is half strength and i only feed every second watering
My BC could be a heavy eater and that would explain why it is only effecting this plant
It looks like it matches up with the issue on the chart

Lockout caused by crazy PH-

Possible because-
I dont have a PH meter just using tap water
Could be a sensitive plant which would explain why its only effecting this plant

Inprobable because-
I have only fed a max of 1/2 str nutes and feed every second watering
Tap water is usually good and if the PH was this out it would probably effect the other plants at least a bit

Heat stress-

Possible because-
The temps have been fairly high at one point close to 90°

Inprobable because-
It is only effecting the bottom leaves
None of the other plants are effected

Bugs-

Possible because
I leave the door open occasionally to let the room cool off

Inprobable because-
The intake and discharge are both filtered
only one plant is effected
doesnt look like bug issues

Also today is feeding day so i would really like an answer from someone who has experience this, if i dont get any then i guess i will go with my previous diagnostic

your stems would be screamin' red/purple, if you had bronzing/necrosis from P lock-out/def. imo

don't feed. less is more. water if due. imo

Ok thanks for the advice, But i havent been feeding much at all... im using a soil-less mix so it has no nutrients in it and since its popped ive fed once with 1/8 str nutes twice with 1/4 str and once with 1/2 str

they are ugly spots fer sure. idk............and i'm totally blazed

Phosphorous deficiency is rare especially in a plant that young. Try a couple teaspoons epsom salts in a gallon of water for your feed and nothing else. Then just water for young plants for a couple weeks. Yes I know people feed and they survive or even thrive but KISS.

B.B.V.C. said:

Ok thanks for the advice, But i havent been feeding much at all... im using a soil-less mix so it has no nutrients in it and since its popped ive fed once with 1/8 str nutes twice with 1/4 str and once with 1/2 str

Click to expand...

it could have gone without any nutes at all, from the size of it

IMO the nutes are the problem

Ok thanks for the replies everyone. Ill just water with plain tap water and maybe add a bit of epsom salt since the amount of mag in the GH lineup is quite low