HELP! Brown spots on fan leaves near top of plant

[heyitsvic]

Hi guys!

This is my first grow and so far so good. It’s just that recently, I’ve noticed that my plant has brown spots on the fan leaves closest to the top three buds. I keep the pH at around 6.0 and a ppm of 600-800 (but recently increased it to 800-1000, thinking that the cause for the brown spots is phosphorus deficiency). My plant is on its third week of flower.

I’m growing in a 2x4 grow tent with a 600W LED about 18 inches away from the top of the plant. The plant sits in BushDoctor Coco-Loco (coco noir) in a 5 gallon pot. The fertilizer used is General Organics (specifically BioThrive Bloom, Big Bud, and CaMg+). I also have two small fans circulating air around the tent and a 4 inch, 190 CFM exhuast fan attached to a carbon filter. The temprature range is 68F - 80F and humidity hovers around 40% - 55%.

As I’ve mentioned earlier, it looks to me like I have a phosphorus deficiency problem. However, Phosphorus is said to be mobile and first appears on the lower leaves… Could my issue be light burn instead, or something else entirely?

Any advise is greatly appreciated, thank you in advance!

[UrbZ]

Phosphorus Deficiencies start at the bottom. You worst leaves would be down there. If your upper leaves are like this and it were phosphorus your bottoms would most likely be dying. Kind of looks like potassium deficiency more than phosphorus.

Check the charts.

[heyitsvic]

Thanks for the reply UrbZ !

Isn't it the same case for Potassium, as it's also mobile (like phosphorus)? The strange thing is that the bottom and middle leaves look fine.

[UrbZ]

Have you checked pH runoff? It could be a high pH issue.

[heyitsvic]

I have been making sure that the runoff matches the pH and ppm that goes into the medium.

[abadtrip]

Raise your led and add more calmag it looks like the led is burning the leaves it's-on the top but not the very top so they have been under the light longer. Just a guess tho

[Chefbjy]

Agree you should raise the light. Light burn can cause other deficiencies.

[heyitsvic]

Yeah, that was my second guess! I'll try doing some LST training on the three colas that are affected, since raising the light will be very difficult.

[Dutchman1]

Maybe lights, cal/mag couldnt hurt.Thats what it looks like to me.

[heyitsvic]

I've raised the lights, did some LST on the affected colas, and raised CaMg just in case. Hopefully the brown spots stop progressing :/

[PRIMO]

I would say it's time to defoliate,you did not say what your feed-water schd. is but I would also add more cal-mag and molasses with water.

[heyitsvic]

Yeah, the plant is quite bushy... I actually did some light defoliation on it last week, but am a bit apprehensive about taking off too many leaves...

Reading and watching videos, I've seen growers take very few leaves to strip the entire plant. I personally decided to start slow (being a first time grower) and defoliate different parts of the plant to see the results.

My current feed-water schedule is to alternate between feed and water every 2-3 days. I will try adding molasses to my feed as well!

I also forgot to mention the strain I'm growing: northern light by Nirvana.

[Dutchman1]

Sounds like you have a good plan, stick with it.

[heyitsvic]

Thank you!

[abadtrip]

If u are getting light burn from your leds I would defol very little if all and wait to see what happens.
If it continues u are going to want more leaves because they will start to die off but they will protect the leaves underneath for a little. Then u can take off the dead leaf
This is last case if u cannot fight off this problem and u need to live with it.
So I gess I am saying wait to defoliated untill u know u won't need the leaves u are going to take.

[heyitsvic]

Yeah that makes sense! I'll hold off on defoliating until things improve. That's a good idea