Rust Spots mid-flower

[pg2052]

I'm maybe a bit confused with what's going on, so I'm going to run through my diagnostic checklist just to share my observations:

1) This is related to an immobile nutrient because it appears on new growth, not on the lowers and spreading to the uppers.
2) This appears to be related to Calcium based on the appearance of rust spots on the leaves and a 'burned' appearance.
3) The plant does not have a Nitrogen deficiency; the plant is showing signs of potential N-tox.
4) pH seems to be stable around 6.3-6.5, both in terms of feed and runoff (can you confirm your instrument is calibrated and/or do another test with drops to verify the color is close to this range through non-electronic means?)
5) There is visible cupping along the leaf margins, indicative of potential heat-stress (DLI measurements rule-out light stress)
6) Runoff EC does not indicate an obvious accumulation of salts in the medium (again, can you confirm calibration of your instrument?)

Possible issues:
Ca lockout due to high pH or N/Mg toxicity?
Heat stress?
Stress due to extremely high VPD (i.e. low RH)?

Hey @low_and_slow, thanks for the analysis. Per your question, I just ran a 3-point calibration on my Apera and everything appears in order. I don't have drops so meter calibration is best I can do at the moment. I just have a cheap little digital TDS Meter (HoneForest). I just tested a cup of distilled water and it registered 25 PPMs. I always thought distilled was real close to 0. Does that give you any pause that it's reading 25?

Regarding heat stress - I can tell you that I was definitely scorching these plants in mid-late veg/early flower. My DLI was close to 70 at one point, as I was running my QB at about 265 watts from the wall at about 20 inches for several weeks. My other plant in the tent started yellowing quite a bit across the canopy when I realized that I was giving them way too much...but the plant in this thread never really showed any signs of heat stress when the lights were cranked. After doing some research I dialed the light way back to about 38 DLI at the top of canopy. Is it possible that this plant is just showing delayed signs of heat stress? The light has been adjusted to something more reasonable for about 20 days - but it definitely looks like the leaves at the top of the canopy are getting fried (so much so that I've realized I'm better off leaving the crispy leaves on just to shade the leaves below -- shaded leaves seem to be pretty much fine).

Regarding the VPD, I've been running RH levels from about 38-48, but there are certainly times when it's been below 38 (but never under 30). I too was wondering if that might be the issues, just because the leaves seem so dried out. I set my humidifier to 55 last night, and turned down the exhaust a bit to try to get a little more moisture in there.

Is it too risky to do a foliar calcium spray at this stage (I'm probably 3 weeks or so from harvest I would guess)?

Thanks again for taking a look under the hood! It was so much easier growing big ass plants on my roof last summer!! Ugh...but I'll get this figured out eventually.

 

[pg2052]

And it’s definitely getting worse too, FWIW. Let’s just came on

 

[pg2052]

I mean “lights” just came on

 

[low_and_slow]

Hey @low_and_slow, thanks for the analysis. Per your question, I just ran a 3-point calibration on my Apera and everything appears in order. I don't have drops so meter calibration is best I can do at the moment. I just have a cheap little digital TDS Meter (HoneForest). I just tested a cup of distilled water and it registered 25 PPMs. I always thought distilled was real close to 0. Does that give you any pause that it's reading 25?

Regarding heat stress - I can tell you that I was definitely scorching these plants in mid-late veg/early flower. My DLI was close to 70 at one point, as I was running my QB at about 265 watts from the wall at about 20 inches for several weeks. My other plant in the tent started yellowing quite a bit across the canopy when I realized that I was giving them way too much...but the plant in this thread never really showed any signs of heat stress when the lights were cranked. After doing some research I dialed the light way back to about 38 DLI at the top of canopy. Is it possible that this plant is just showing delayed signs of heat stress? The light has been adjusted to something more reasonable for about 20 days - but it definitely looks like the leaves at the top of the canopy are getting fried (so much so that I've realized I'm better off leaving the crispy leaves on just to shade the leaves below -- shaded leaves seem to be pretty much fine).

Regarding the VPD, I've been running RH levels from about 38-48, but there are certainly times when it's been below 38 (but never under 30). I too was wondering if that might be the issues, just because the leaves seem so dried out. I set my humidifier to 55 last night, and turned down the exhaust a bit to try to get a little more moisture in there.

Is it too risky to do a foliar calcium spray at this stage (I'm probably 3 weeks or so from harvest I would guess)?

Thanks again for taking a look under the hood! It was so much easier growing big ass plants on my roof last summer!! Ugh...but I'll get this figured out eventually.

25ppm might have come from dirt inside the cup or whatever, it's not enough to create a problem like the one you're seeing if you're also following a pretty standard fertilizer regimen.

I definitely wouldn't try to correct this with a spray right now, unlikely to make a big difference since the underlying problem appears to be making things worse. Immobile nutrient lockout/deficiency won't repair itself like a mobile nutrient would.

My guess is that the plant can tolerate a ton of light and probably a good amount of heat, so it didn't stress as badly as the others, but now in later flower it's starting to show signs of problems due to low RH.

The checklist for plant growth includes the following inputs:

Light
Leaf Temp
Leaf/canopy RH
Wind/air movement
CO2 concentration
Nutrients in the root zone
Water in the root zone
Oxygen in the root zone
Temp in the root zone

If I have to identify which of these might be causing a problem, my thought is that RH/Temp in the canopy are likely placing your plant in very high VPD conditions, which along with high light intensity if forcing a rapid uptake of water and nutrients from the root zone. Because the leaf temp and RH are likely out of range for optimized metabolism, what you're seeing looks like N/Mg related lockout of Ca. The plant just can't make more cells productively until all the above parameters are in balance. If you push a plant that's out of balance like this, toxicity-related lockouts like this can pop up as a result.

If that were my plant, I'd aim for 'light' feedings below 500ppm and try to give them more often, with the goal of keeping a highly oxygenated root zone and using waterings to slightly increase/stabilize RH. I'd also set your humidifier to stay in the 50-65% range for a little bit to see if that helps.

You could also try to just coast on pH adjusted water for a while and try to see if you can get the bottom row of fan leaves to start to fade a bit. If you start to see the nitrogen leach out and the plant begins looking healthy again as a result, you can lock-down N-tox as your cause of Ca lockout.

If you haven't watched this video yet, you should take a look:

 

[pg2052]

If that were my plant, I'd aim for 'light' feedings below 500ppm and try to give them more often, with the goal of keeping a highly oxygenated root zone and using waterings to slightly increase/stabilize RH.

Should the light feeding just consist of bloom nutes ( Tiger)? Definitely stay away from Calcium/Magnesium?

Thanks!

 

[low_and_slow]

Should the light feeding just consist of bloom nutes ( Tiger)? Definitely stay away from Calcium/Magnesium?

Thanks!

Yeah I would only be adding a base nutrient right now, no additives.

Until you feel entirely confident harvesting a crop you're proud of, I would advise any gardener to rely on a single base-nutrient product. I'm really confused at how many people are feeding a cannabis specific fertilizer AND a calcium and magnesium supplement. You should only add a supplement if you have a clear plan for pushing your cultivar, or an obvious deficiency that can be corrected with an amendment. Most problems people experience on autoflowers are lockout, not true deficiencies from the absence of a needed micro/macronutrient.

 

[pg2052]

Yeah I would only be adding a base nutrient right now, no additives.

Until you feel entirely confident harvesting a crop you're proud of, I would advise any gardener to rely on a single base-nutrient product. I'm really confused at how many people are feeding a cannabis specific fertilizer AND a calcium and magnesium supplement. You should only add a supplement if you have a clear plan for pushing your cultivar, or an obvious deficiency that can be corrected with an amendment. Most problems people experience on autoflowers are lockout, not true deficiencies from the absence of a needed micro/macronutrient.

Yeah I get that. I probably got a little too amped up about getting my grow going and bought a bunch of stuff that I kept hearing folks talk about. I'll scale it back considerably for next grow. I'll just stick with the fox farm trio, and will probably cut the Ocean Forest quite a bit more this time around.

 

[low_and_slow]

Yeah I get that. I probably got a little too amped up about getting my grow going and bought a bunch of stuff that I kept hearing folks talk about. I'll scale it back considerably for next grow. I'll just stick with the fox farm trio, and will probably cut the Ocean Forest quite a bit more this time around.

It's interesting for me to see because I haven't been really active on here for a while; coming back from other forums, I think you see folks adding calmag supps a lot less by default elsewhere and here it seems common practice. A lot of fertilizers use Ca(NO3)2 Calcium Nitrate as a Nitrogen additive, so if your fertilizer has that it's fairly unlikely you'll need additional. I'm always leary of a "calmag" supplement as well since depending on a variety of factors you may want to add Ca and Mg in different ratios to hit your target, especially if you amend your medium with something like lime or gypsum.

 

[pg2052]

It's interesting for me to see because I haven't been really active on here for a while; coming back from other forums, I think you see folks adding calmag supps a lot less by default elsewhere and here it seems common practice. A lot of fertilizers use Ca(NO3)2 Calcium Nitrate as a Nitrogen additive, so if your fertilizer has that it's fairly unlikely you'll need additional. I'm always leary of a "calmag" supplement as well since depending on a variety of factors you may want to add Ca and Mg in different ratios to hit your target, especially if you amend your medium with something like lime or gypsum.

The CalMag suggestion I got from another forum (ILGM) that I spent some time on before moving here. And when I created a "what am I missing to get started post", this was one of the first responses I got by someone on that board that was seemingly very experience with autos.: "CalMag calcium magnesium supplement, You will need it at some point with the products. Not a knock against (I use it) just how it is"