Please help getting white leaves and curling close to harvest

Hello. I am growing indoors. 600 hps and fox farm soil. I have been doing it about two years and have had problems with spider mites but other then that had some great harvests. recently I am noticing my plants that are getting about 6 to 8 weeks along are getting curling leaves and yellowing and brittle dieing leaf tips. I am using the same fertilizer on all the plants and only the older ones are doing this. i am spraying with safers spray for mites but that is not the problem since all of the younger girls are fine.
I enclosed a picture of a plant that got so bad I took her outside a week ago. is this a disease? It is not too hot in my grow room. Please comment if you know anything. I could send a picture of the other plants and one that is starting to do the same thing if that will help. Thanks.

Did this plant even have pistils? All I can see are the Calyxes that look seeded to me. I also see very little trichomes. It looks as though the remaining Nitrogen has been sucked out of those leaves in the final production but I've never seen that happen like this. I'm sub'ed to see what we can learn.