What could be causing this?

not over spray and I say this as a certified expert after freaking out 72 times thinking everything from mold to mites
@Keffka nailed I never seen the spots before but they are a trait as far as getting rid of it I would do a good Flush with luke warm water if you are using soil then when it drys out start your nutes back on but I would cut them back for a little bit. what kind of Nute program are you using?. the guys are also right about using a foliar spray with the lights on, was the spray Potassium silica ? does it need to be diluted? these are things I would do But there are better growers than me they might recommend something else
the reason I asked about Potassium silica is the spots look like what I use before I dilute it, Potassium silica and NEEM is the only spray I use and thats only with lights out check out the show

Growing Belushi

Thats where I learned to spray in the dark lol.
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Best of luck and Grow on

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How do you recommend rectifying the problem.

Potassium is rough, especially in flower. Once you see it like that you’re likely going to play catch up for the rest of the grow. This isn’t the end of the world but it will affect your yield.

If you’re using a specific line of nutrients there’s a chance you can pull it back from the edge if you can get the levels correct. @Savvage61 gave some pretty good information but we do need to know a little more before we can definitively say one way or the other.

What line of nutrients are you using? Advanced Nutrients is the company but they’ve got several different nutrient lines such as Micro, Grow, Bloom, PH Perfect versions etc. Which feeding schedule are you using, and where at in the schedule are you?

I grow photos so I don’t want to recommend you throw a bunch of potassium at your plant if you’ve got 2-3 weeks left on the grow so I’ll tag @Grand Daddy Black and @farside05 since they’ve both grown some of the most gorgeous autos I’ve ever seen and see what they have to say.
 
advanced nutrients in plain potting soil mix. 15 ml in a gallon of water phed at between 5.8 and 6.8 with 1/2 tsp of cal/mag and sicilca . bestva 2000w led with 205w pulled fron wall. Thanks for the reply
You are in soil, so pH needs to be more between 6.2 - 6.4. 5.8 is too low & 6.8 is way too high. Not saying this is your problem, but your pH is all over the place. Who knows what the soil pH is at this point.
 
If @Hebrewdiver is using AN, at their recommend levels, there's probably something else going on. AN has one of the highest levels of K of all the nute manufacturers. I did an elemental breakdown of the most popular nutes over HERE. AN is in the 280ppm range. Adding additional K beyond that should be undertaken with great caution, if at all. When members were playing with adding supplements to Mega Crop, also higher in K, there were some catastrophic failures when K got in the 300-320ppm ranges. Plants turned crunchy overnight.
 
Did you spray with light on? Never spray with lights on.

I fully understand your reasoning and respect your knowledge on this, so could you answer me this related question?
I never spray anyway - so far - but if I did, I'd be worried about spraying with the lights off from a mould / rot perspective as much as I'd worry that spraying with the lights on could cause burns where the specular highlights hit the plant.

Is the rot not an issue because you'd not spray while the plants have developed buds anyway, or some other reason? Would you spray a while before lights on or after lights out. Thanks for any explanation that will help me understand this better.
 
If I ever need to spray for any reason I find doing it at lights off gave best results. There was very little water spots on leaves and my fans keep air moving to resist any mold. Good air flow in your grow area is very important, especially if you have frequent watering events taking place.
 
Browning of the leaf edges... I'm gonna guess potassium deficiency. My approach would be to fertigate with solution grade potassium sulfate (K, S), or solution grade Langbeinite (K, Mg, S). My custom soil mix already has enough K, so I only fertigate like this in late veg and flower. I also use a high-N liquid fert as needed, NPK 11-2-4.

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good luck and happy growing! 🌱
 
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