My dad had been growing for awhile now but he is running into this on a few of his plants. He hasn't changed anything from previous successful grows. Water, soil, nutrients is all the same and fresh.
These were started indoors until about two foot tall and planned outdoor. Zone 5B. Watered daily, no water logging off soil. All three were started from seed and all the are different strain. My dad is very experienced with this but I am not. If I left anything important out please let me know and I'll get the info from him.
I second this opinion. It looks like the early stages of nutrient burn. It could be as simple as not mixing the nutrient solution enough so the bottom of the barrel is hotter than the top. My first suggestion to every grower is always less is more. I like to use super soil up until flower for that reason specifically, and then just use supplemental nutrients twice weekly until flush. Unless you are growing commercial crops on giant plants or are in a CO2 supplemented environment then it is unlikely the plants are getting large enough to require large amounts of supplementation.