At that stage of the grow, I automatically think salt build up and nutrient lockout. (It doesn't look as spotted/rusted as Ca def.).
I’m growing in soil, Michigan M3. I haven’t really fed much other than URB Natural and aloe. I use straight tap that I let sit out to de-gas and I don’t check ph. I have minimal runoff as the plants are in smart pots. It’s the plant in the top right cornerAt that stage of the grow, I automatically think salt build up and nutrient lockout. (It doesn't look as spotted/rusted as Ca def.).
If you're growing in soil, what kind of soil and nutrients, supplments? How much do you feed? How much runoff do you typically have? Can you measure the PPM of the runoff?
I'd like to see photos of the entire plant(s), under normal lighting.
[Note: I typed this post before seeing subsequent posts advising against Ca def. I thought I'd go ahead and post it since you think Ca def could be a risk in your environment. This information might help if you ever do need to supplement Ca.]QB’s are known to cause plants to require more calcium than hps or cmh bulbs.