Hey man, first off, if you have no pests, what food are you feeding and at what pH? It looks like low on potassium, magnesium, maybe even a little boron deficiency. I don't think it's caused by too much of one thing as the plants look healthy, but I do think either an increase in flowering food is now needed or pH must be adjusted. It is healthy, if no more food is available they will feed off their own leaves and that is why they start to die. I would check soil pH, if that is in range then I'd feed a full dose of nutrients, or a little higher if that is what you were doing. pH it and water to runoff. Test the pH of that runoff right away too if possible to make sure it's pretty well coming out the same as it went in. This should flush it out with some clean nutrients and I would think will take care of your issue. Maybe you are feeding enough though and the pH is just off so testing soil, plus run off will tell you if that is the case. Hope it helps man, happy growing!
Hey DandyLB, I just had this exact same issue this grow. For me: 1) I was low Nitrogen, but yours appear lush green. 2)Being a cheapo, I was using Epsom Salt dissolved 1/8 tsp per Gallon and Oyster Shell Powder for Calcium. I finally bought Cal Mag Plus and my selective splotchy leaves vanished. I ALWAYS had what you are seeing, but it only happened to older leaves. One of yours that is 100% crispy is up near all lush green every other leaf around it.
So, when in doubt, besides flushing it out, and reassessing PH, also just buy the Cal Mag!
I have a theory that slight deficiencies reduce the life span of individual leaves, just as Zolt' advises plant takes from the old to feed the young growth when it has to. I noticed when fighting white powder mold w 1 tsp 35% H2O2 per Quart water spray, that new growth cared not. The oldest lower leaves got splotchy as yours kinda show.
When in doubt..... flush it out.... I would use something with enzymes so it thoroughly breaks down any salt deposits..... most nutrient schedules include recommended flushing intervals which are VERY commonly overlooked.....it looks like you are doing fine with your feedings and must have a decent handle on ph so , if these were my girls....I'd flush em' out and continue doin' what you doin'