Looks like the start of Calcium deficiency. Are you giving it any Cal-Mag?
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Strain; Type, Bag seed, or NA
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Hey @Deepsix a couple weeks ago I would have suggested the same diagnosis but since then it finally sunk in that calcium is an immobile nutrient and when it starts to show it will show on the upper new growth areas of the plants tops etc as it can’t be reallocated from old growth to new growth to make up the deficiency or lock out due to soil PH etc.
Also I treat calcium and magnesium as separate issues so using something like a CaMg all in one mix is feeding both when many times only one or the other is really needed. Magnesium is a mobile nutrient and will show more on the older larger fan leaves first as the plant will take Mg from those and redistribute it to newer growth. At least this is how I currently understand things and that’s changing as I go and grow.
Hope this helps