I hate to see sick plants, especially when they were so healthy and thriving.
I have read through the recent part of the journal and listened to you all. Good info being discussed.
If this were my grow, I would make some ph7 water up, lightly pour it on everywhere, evenly spread so you don't get any dry pockets, then wait half an hour. Then flush. Drain them properly.
Then whilst they are draining, back the lights off a bit. Just give them some time to recover. Have all the lights like 3ft away for a week. Try and raise rh slightly whilst they recover.
If you are feeding your plant say ph 6.4 and your run off is 6.3, maybe try feeding them ph 7 instead of ph 6.4. Reason being if the ph drops and your run off is 6.3, you maybe be able to get it to drop from 7 to 6.3. That would then help some of the vital nutrients that get taken at higher ph levels to be used. There are mobile and immobile nutrients that have a peak uptake ph level. There is a chance your plants are not uptaking the nutes closer to 7. 6.3 - 6.4 sounds close to 7, but it ain't really in ph terms. If your ph drops when you feed, start high at 7. If it rises when you feed, Start low, like 6.3
To me, the grow looks like a bit of light burn. Can a plant handle that much light at that distance if it's in perfect health? Yes. If it's not in perfect health, maybe not. It probably want a cloudy day and rain tbh.
Backing the lights off will give the plant a chance to spend a few extra days drinking the water. I'd even consider turning off the hps for 48 hours and just running the cobs.
The plants are probably drinking quite fast, maybe drinking water faster than they are uptaking nutrients, then causing toxicity in turn.
Recently I grew 3 auto's and did a very heavy bat guano feed, ended up burning the hell out of them. Then suddenly got crispy leaves, curling up, same kind of patches that looks like light bleaching. After flushing the plant was almost immediately back on track. The buds turned out nice.
Definately give them a few days off and plenty of water. Get some things that are immediately avaulable like epsom salts, worm castings teas, sea weed etc. Thing with low n-p-k that plants really like.
In England we have alg-a-mic which is mainly sea weed. Something like n 0.1 p 0.1 k 0.1 But the plants love it! You can get them right back on track. Adding something like this would stop the plants yellowing after the flush. I'd normally add one of the listed things 24 hours after a flush. At a very light dose too.
