Whats this rust color on leaves

jlinzmaier · Jul 11, 2025

Hello.

I'm rather new to growing. Ive noticed this rusting sort of change to some leaves. It started a week or two into flower and continues to show up. Im using cronk nutrients. Is this an excess salt/nutrient build up issue? I'm now using nutrients every other watering and two weeks ago I flushed it with 4 gallons to remove any build up. Maybe it isn't even worth worrying about, but I like to learn and id love to hear anyone's thoughts on what this is.

This is a meatbreath auto currently at day 62.

Thank you Kindly!

neneboob · Jul 11, 2025

My guess is PH lockout, or Potassium deficiency. I'm not super well versed on what you're using to feed, but if it's got Potassium I'd guess salt buildup is causing a high PH, so flush. If cronk doesn't have potassium the plant needs some added to the mix.

Eledin · Jul 11, 2025

What medium, what ph, what water (tap, well, ro...) and are you using the calmag from the same line of nutrients?

ArtfulCodger · Jul 11, 2025

Without knowing anything about how you're growing, these pics look over-lit (edge curl and tip twist), over-watered (leaf texture), and over-fed (tip/edge burn, dark color). The brown lesions themselves are a nutrient availability issue, but it's tough to say what's causing it without knowing more about the grow.

Eledin · Jul 11, 2025

To me it looks like calcium deficiency but yes the top leaf seems like its been a bit hot because of the edges going up but the marks are in the lower leaves too. What light and at what distance? Add those two to the other questions hahaha.

jlinzmaier · Jul 11, 2025

Ok. Let me try to provide more detail.

Sadly I used miracle grow potting soil for this plant. I hate to admit it and that may be part of the problem.

Using half ro water and half tap water for some pH buffer capacity. And using the full line of cronk nutrients including calmag.

I pH adjust water to 6.0 -6.2 (5.8) in earlier growth.

The light is an old optic 6. Currently about 24 inches above the plant. No par meter to give anymore info there. New spiderfarmer se7000 is on the way.

When I first noticed this I checked pH of runoff and it was very low at 5.2. I suspected some sort of nutrient build up and after the flush the run off was back up to 5.8. I didn't adjust further to not cause any shock from pH adjustment. Recent run off pH was around 6.0.

Again, I'm just a beginner here trying to learn. This is only occurring on about 8 or 9 leaves. The remainder of the plant looks healthy to me but I guess I'll take more pics and get input. Had no idea I was doing so much wrong.

Jeremy

jlinzmaier · Jul 11, 2025

Also, its in a 7 gallon fabric pot - way too big. I get that now, just too late to go back. That may be where the overwatwring issues are coming from. I'm now using 4 gallon fabric pots.

Thank you again for all the input.

Eledin · Jul 11, 2025

Its not a major problem specially in late flower, sometimes you will get deficiencies.
"When I first noticed this I checked pH of runoff and it was very low at 5.2"
Usually its the obvious thing and in this case it was the low ph probably combined with that type of soil. You only noticed the low ph once it started showing problems, if it doesnt progress now that the ph is adjusted then it was that 100%. If it keeps progressing it might be something else. Your line should have everything the plant needs. PH below 5,5 will lock out calcium in soil so thats why it might have a calcium deficiency. As for the light, what model and at what distance? The edges of the upper leaf are curled upwards, many times is a sign of heat or light stress.

Eledin · Jul 11, 2025

Also the tips of the upper leaves are slightly yellow while the rest are not. Thats totally harmless as long as it doesnt go further than that, everyone pushing max light intensity gets them, but combined with the curled leaves might develop into a problem if the light is too intense/too close.

Eledin · Jul 11, 2025

Also I planted autos in 5 gallons before, I think even 7, but considering the average height of the plant. If you choose a small auto then 5 gallons might be too much but you can definetely grow autos in 5 gallons without watering problems. Even small autos will probably still colonize all the soil and grow short but chunky as hell.

Eledin · Jul 11, 2025

Also make sure to let the water you use from the tap sit outside in a jug for at least a few hours. At the very least it will have chlorine and it needs to evaporate, if you check for a local report of your water online you can check if it has chloramine, in which case I would treat it or just use RO water since youre giving a calmag and the bases should have NPK and all needed minerals according to them.

Eledin · Jul 11, 2025

For treating water with chloramine you can let it sit outside for a day if you have direct sunlight so the UV breaks it down or you can let the chlorine evaporate and then add a dash of hydrogen peroxide and let it degrade for a few hours. You let the chlorine evaporate first because otherwise you will need more hydrogen peroxide as it reacts with both, but chloramine doesnt evaporate on its own. In a few hours the hydrogen peroxide and the chloramine will degrade and release gases and only some ammonia will remain in the water which your plant will convert into nitrogen.
Chlorine and chloramine can cause problems in the roots and also spots on the leaves. Tap water is very different depending on where you live, if its heavily treated due to sanitary concerns it will have more chlorides.

jlinzmaier · Jul 11, 2025

Thank you all very much!

jlinzmaier · Jul 11, 2025

I'm using a 600 watt optic 6 LED thats about 7 yrs old and has about a 6mo to a year of use. At a height of about 24+ above the top of the plants. Either way it was time for an upgrade so I'm looking forward to getting the se7000 and adjusting brightness accordingly

GeorgeJ10 · Jul 11, 2025

jlinzmaier said:
Hello.

I'm rather new to growing. Ive noticed this rusting sort of change to some leaves. It started a week or two into flower and continues to show up. Im using cronk nutrients. Is this an excess salt/nutrient build up issue? I'm now using nutrients every other watering and two weeks ago I flushed it with 4 gallons to remove any build up. Maybe it isn't even worth worrying about, but I like to learn and id love to hear anyone's thoughts on what this is.

This is a meatbreath auto currently at day 62.

Thank you Kindly!

Looks like a classic case of early potassium deficiency or light nutrient burn, common in early to mid flower. Since you flushed and are now alternating feedings, you're on the right track. Just monitor new growth and avoid over-correcting. You're doing great!

Whats this rust color on leaves

Whats this rust color on leaves

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