Brownish red spots early flowering

hello there. I started flowering 3 plants on February 28th, they had been in veg since January 19th. I started them in 50/50 pro mix and fox farm ocean forest. I have them in 3 gallon containers with a 1000 watt HPS set to 75% power. On Monday I added a "light feeding" of "general organics bio thrive bloom" along with one tsp of "roots organics elemental" for cal/mag.
When I went to go check up on them today one of the three plants had some odd reddish brown spots on several of the fan leaves. Does anyone know what this could be? There was some light burn (I think) on one of the new growths on that plant so wondering if it could have to do with stress? No signs of the spots on the other two plants and he y'all received the same amount of nutrients.
Thank you guys so much!

Update: all of the leaves that started getting the spots are turning yellow/brown and some parts black. Definitely dying but I haven't seen new leaves starting to get the spots yet thankfully. Only seems to be affecting the fan leaves.

HMMM...me thinks Manganese or Potassium...take a look here..http://www.growweedeasy.com/manganese-deficiency-cannabis

and here:http://www.growweedeasy.com/potassium-deficiency-cannabis

Good Luck and Great Growz

If they have been in 3 gallon pots since January feed them, although fox farms may be consider hot by some, in that size pot you should have started nutes after 3-4 weeks.

Okay thank you guys for responding. Do you think I should feed them everything I would as if they were vegging should I stick with my "bloom" specific fertilizer since they are flowering?

Since you are only in the 2nd week of flower, you will still want to make one or two more waterings with a veg nutrient. Around wk 3 is a good time to transition with a veg/bloom, then right before the stretch is complete, around wk 4, start with the bloom.
They've been in the container since January with half promix, which has little nutritional value.... I bet the plants are hungry.

I would agree with the light stress, probably too much light abruptly. Is the affected plant receiving the most intense light?

Start at half strength and go from there., but you should be feed-water- feed or feed-water-water.

The affected plant it off to the side a little bit, not directly under the light. Okay! Thank you guys so much for the help. I will get some veg nutes immediately and give that a try.

Update: still having issues with said plant.... I tried adding veg nutes and the problem is still progressing. About half the leaves are getting the spots on them now ):
I took a PH reading and my water is about 7, after I add the nutrients however it drops to around 5. Could this be a PH problem after all? The other two plants in the exact same conditions look just fine....
Should I attempt to flush and reset soil PH maybe? I will attach some new pictures to show the end result of the problem leaves. Thank you guys so much for all your help, I really appreciate it.
Also, the plants are still in 3 gallon pots. Not sure if that is a normal size for the amount of time I am giving them but I am 90% sure that the plant is root bound at this point.

Hard to reset the pH in soil, and a pH 5 is way to low for soil. I had a few runs in FF OF and ran into the same trouble, not enough lime in the soil to buffer the pH.

boilingoil said:

Hard to reset the pH in soil, and a pH 5 is way to low for soil. I had a few runs in FF OF and ran into the same trouble, not enough lime in the soil to buffer the pH.

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Would flushing with a ton of water and re adding nutrients with "PH up" help at all or would I just be concentrating salts?