Brown Spots - Nute Burn or LED Damage

[InsaneMembrane]

Would love some assistance at diagnosing this one.

a) I’m thinking it could be nute burn as these were topdressed with guano meal 3 days ago and watered thorougly. Guano is 5% N - 12% P - 3% K and 28% Ca, 2% Mg and marketed to show effects in 48hours. I did add 50g to a 25L pot.

b) Or it could be LED damage because they have stretched into the lights and were sitting at 900 PPFD. I raised the light so they’re currently sitting at 750-800ish.

Normally i would disregard it as the plants look good but this time I want to be certain of what it is hoping it’s not an indication of something worse in the upcoming weeks.

cheers

 

[Creeperpark]

If its leaves are on the top of the plant closest to the light are bronzed, I would consider possible light burn. Your plants look too good for nutrient burn, and I just don't see that happening now. Nutrient burn will most always start at the tips first and then move to the leaf and your tips look good. You may want to keep an eye on it and watch it closely over the next couple of weeks.

 

[MrFancyPlants]

This is almost certainly a Calcium issue, which in my experience seems to affect how much light a plant can process before it gets burned.

Ca is immobile and therefore must be available for uptake in the root zone at the time of tissue growth. So, you may be seeing a problem that actually occurred when the leaf was being formed, rather than an ongoing issue. In any case, you're looking into the past.

Per Jorge Cervantes and the Cannabis Encyclopedia, your possible causes for availability issues are:

- acidic soil;
- excess NH4+, Mg, K, or Na;
- environmental issues that impair transpiration, e.g. low/high humidity;
- irrigation issues (in my own experience, underwatering can easily cause this).

Per my own experience, availability of Ca is critical during the stretch, as the plant is forming a ton of new biomass, and you don't see issues until all this biomass gets to properly cook under the LEDs for a week or two. I've been burned (pun intended) enough times, and my soil mix has been continually adjusted to include more and more calcium-heavy amendments, such as crab shell, egg shell, and oyster shell. Of course, the impact of long-release additions is questionable if you don't recycle your medium.

By the way, your guano sounds like a fertilizer mix rather than pure guano. What are the ingredients? I'm curious where K/Mg/Ca are coming from.

The plants do look great, by the way. If the spotting doesn't get worse, I wouldn't worry about it. Good luck!

 

[InsaneMembrane]

This is almost certainly a Calcium issue, which in my experience seems to affect how much light a plant can process before it gets burned.

Ca is immobile and therefore must be available for uptake in the root zone at the time of tissue growth. So, you may be seeing a problem that actually occurred when the leaf was being formed, rather than an ongoing issue. In any case, you're looking into the past.

Per Jorge Cervantes and the Cannabis Encyclopedia, your possible causes for availability issues are:

- acidic soil;
- excess NH4+, Mg, K, or Na;
- environmental issues that impair transpiration, e.g. low/high humidity;
- irrigation issues (in my own experience, underwatering can easily cause this).

Per my own experience, availability of Ca is critical during the stretch, as the plant is forming a ton of new biomass, and you don't see issues until all this biomass gets to properly cook under the LEDs for a week or two. I've been burned (pun intended) enough times, and my soil mix has been continually adjusted to include more and more calcium-heavy amendments, such as crab shell, egg shell, and oyster shell. Of course, the impact of long-release additions is questionable if you don't recycle your medium.

By the way, your guano sounds like a fertilizer mix rather than pure guano. What are the ingredients? I'm curious where K/Mg/Ca are coming from.

The plants do look great, by the way. If the spotting doesn't get worse, I wouldn't worry about it. Good luck!

Good point there, thing is the current mix has double the amount of bone meal than last time, reason being I wanted to up Ca and P by a lot. Like almost 2 cups per cft (0,5kg per 70L soil). On top of that they are being treated weekly with a CalMag+ foliar (Ca, Mg, Fe, Mn, Zu).

these are currently at 22 days of flowering.

regarding guano, they’re the numbers the guano company claims, it’s just bat shit and nothing else and they got all those organic and ecological certificates done etc. The numbers have to reflect the diet of the bats, i dont think its weird seeing those numbers.

 

[GoatCheese]

Yea, i’d say you’re on the limit with light intensity or just over it..and i think the plant might have dried up when that leaf damage happened. Those leaves have a little dried out-look to them, not badly but abit; they lose their waxy shine abit and may feel little dry in your fingers when the light is abit too much for them and/or the plants dry out too badly.
= the led radiation damage is caused by the radiation dehydrating the leaves (leaf surface temperature problem) and it gets worse if the plants are under watered/dry out badly. ..especially during fall/winter seasons when the air is dry already.

Plants do better under modern leds when they’re hydro and coco grown vs. soil, cause in hydro-style growing there is naturally more moisture in the plant tissue compared to soil grown plants. So if you’re growing in soil you have to dim your lights down abit compared to coco/hydro growing.


I’d say raise your light fixture 15-20cm more and see how that goes but i’f you can’t raise your light then i’d dim the light down 10-15%

People give their plants extra CalMag which will help keeping those leaves in better shape under modern leds. I give my plants 0,3ml/L of Canna CalMag (soil grow) and some Epsom salts once in 3-4 weeks.

 

[InsaneMembrane]

Yea, i’d say you’re on the limit with light intensity or just over it..and i think the plant might have dried up when that leaf damage happened. Those leaves have a little dried out-look to them, not badly but abit; they lose their waxy shine abit and may feel little dry in your fingers when the light is abit too much for them and/or the plants dry out too badly.
= the led radiation damage is caused by the radiation dehydrating the leaves (leaf surface temperature problem) and it gets worse if the plants are under watered/dry out badly. ..especially during fall/winter seasons when the air is dry already.

Plants do better under modern leds when they’re hydro and coco grown vs. soil, cause in hydro-style growing there is naturally more moisture in the plant tissue compared to soil grown plants. So if you’re growing in soil you have to dim your lights down abit compared to coco/hydro growing.


I’d say raise your light fixture 15-20cm more and see how that goes but i’f you can’t raise your light then i’d dim the light down 10-15%

People give their plants extra CalMag which will help keeping those leaves in better shape under modern leds. I give my plants 0,3ml/L of Canna CalMag (soil grow) and some Epsom salts once in 3-4 weeks.

Thanks for your input and yes I am keeping these slightly underwatered which would confirm what you said. Might also be strain related, how much DLI they prefer as im not seeing these spots on other strains. I’m also supplementing with CalMag+ foliar weekly due to LEDs.

 

[Koondense]

All that was mentioned plus check your vpd.
I had similar burns with high temps and too low humidity.

Cheers

 

[GoatCheese]

Thanks for your input and yes I am keeping these slightly underwatered which would confirm what you said. Might also be strain related, how much DLI they prefer as im not seeing these spots on other strains. I’m also supplementing with CalMag+ foliar weekly due to LEDs.

Yea, some plants/strain fair better under modern leds than others. Some can take it really well but some you have to keep farther away from the light and make sure they won’t dry out too badly

 

[InsaneMembrane]

All that was mentioned plus check your vpd.
I had similar burns with high temps and too low humidity.

Cheers

Should be fine, 30°C and 60% rH, but I don't have a way to measure leaf surface temps to know the exact VPD.

Yea, some plants/strain fair better under modern leds than others. Some can take it really well but some you have to keep farther away from the light and make sure they won’t dry out too badly

Will experiment a bit with light distance and dimming. Problem is if I dimm I lose temp. (25°C) but if I don't dimm and only raise the light I can't go lower than 800 PPFD but the temps stay in my preferred 30°C range for LEDs.

Curious what DLI or PPFD was most consistent for you or for anybody reading this thread ?

 

[Switcher56]

FWIW I go by tent temp. Your VPD is where it should be IMO. Drop 100 umols on your light and see how they fair out

 

[f-e]

Odd timing, to get a Ca def just after giving some. It's worth noting that Ca and Mg can cause almost identical spotting, with the colour separating them. Ca is rust like, while Mg a loss of colour. I'm not actually sure which I'm looking at. So after a few days have read your tale of woe in detail. Finding cause to reply

28% Ca, 2% Mg

That's not the typical 2:1 ratio. The net result of such a feed might be less Mg availability. I expect it would be transitory though.

 

[flylowgethigh]

Dry soil is bad juju, IMO. I like it good and wet so the microbes and ions can move about.

 

[InsaneMembrane]

Dry soil is bad juju, IMO. I like it good and wet so the microbes and ions can move about.

These weren't left bone dry, just slightly underwatered as I was away a day longer than my usual watering schedule. Also I'd be careful with "good and wet", it can easily lead to "pain and horror", cold temps or high rH etc.. I can show you the horrors of overwatering as I'm a recovering addict of overwatering plants.

Odd timing, to get a Ca def just after giving some. It's worth noting that Ca and Mg can cause almost identical spotting, with the colour separating them. Ca is rust like, while Mg a loss of colour. I'm not actually sure which I'm looking at. So after a few days have read your tale of woe in detail. Finding cause to reply

That's not the typical 2:1 ratio. The net result of such a feed might be less Mg availability. I expect it would be transitory though.

I know exactly what you mean and I do usually spot interveinal discoloration when Mg is lacking but I don't think this is it. Lack of Mg would be more "even" too from my experience. Also keep in mind I'm applying a weeky foliar with:

• Nitrogen (N): 4%
• Calcium (Ca): 3.2%
• Magnesium (Mg): 1.1%
• Iron (Fe): 0.09%
• Manganese (Mn): 0.05%
• Zinc (Zn): 0.05%

I'm leaning towards a temporary bad response to high DLI (40 mols) as these have stretched very close into the light, sitting at over 900 PPFD for a couple of days, no CO2. I have since lowered it to 750ish (33 mols) and haven't observed any more brown spots since. And as I've mentioned they were slightly underwatered which definitely could inhibit Ca uptake. Keeping an eye on it though for sure.

 

[GoatCheese]

Curious what DLI or PPFD was most consistent for you or for anybody reading this thread ?

Part of the solution is that you start watering your plants abit more so that there is more moisture in the plant tissue and not letting them dry out too badly.
If you dim your leds down abit can you turn your exhaust ventilation down abit too? which would help to keep the temps up and to have higher humidity % in there.
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I don’t have any meters, i just play with the dimmer in my flowering tent cause i can’t raise the light fixture any higher. 160cm tall tent. I have three 75 watt Cree cobs in there. i’m running them with 25-30 watts per cob depending how tall my plants are. The cobs are 50-70cm above canopy depending.

A dimmer on your grow light is a must but a meter isn’t, imo, once you have used your lights for few grows (=messed up a few plants) and get to understand what’s going on cause modern leds are quite different compared to a HPS light. The radiation is quite intense and they need some distance to canopy, more than many led grow light companies seem to recommend. But once you have your leds/cobs dialed in for your set up the results and energy saving are impressive.


What i have noticed is that dry cold winter season is a bit problematic with modern leds/cobs and how easily the leaves start to get dehydrated/fried under led radiation, so i’ve learned i have to dim the cobs down abit compared to humid summer season and maybe give the plants just a bit more calmag.
..i also had to buy a ventilator fan with a controller on it so i can set the ventilation as low as i can for winter season so that the cobs can warm up the tent a little to get the humidity rising abit.

 

[GoatCheese]

Dry soil is bad juju, IMO. I like it good and wet so the microbes and ions can move about.

Yea, yields may suffer quite a bit if you under water too badly, and led radiation will bite your plants more easily..