i know this is an old thread but..
its temp combined with overwater..
Cannabis, especially og's need the hotter temps. This is because calcium is very important to canna and og strains, but calcium is immobile so it cannot be moved from one part of plant to another. That means if your light is strong, but your temps are really cold (plant doesnt need to cool down as much) and your rootzone is cold(plant needs less water to cool itself), your plants are photosynthesizing full speed, but not uptaking (because they dont need to) enough water to provide necesary calcium to keep up with new growth. You can either raise your calcium nitrate or other cal sources to dangerous high levels in the rootzone,-- which will cause lock out issues--, or raise temps (rootzone and canopy).
Low humidity compounds this problem (signals plant to close stomata thus reducing water loss >> and UPTAKE), as well as c02 supplementation, because it makes photosynthesis easier. This is why co2 rooms are recomended to run hot (low 80's)
JUst my opinion, and it may change in the future, but i think the optimum way to grow is keep temps almost as high as possible within reason (for og i run83 veg 79-80 flower). This allows you to feed at a much lower strength more often because nutes are being drawn up in the extra water your plants use in the high heat. I think this makes flushing in the end easier and overfeeding less likely if you know your stuff..
