rolla * is dead on. To me your plants do not like the 6.1 ph, soil bro ineeds to be 6.7 to slighlty higher. you have severe nute lockout and nute toxicity due to the low ph and high ppms. coupled with heat stress. This will leave the rust clolored brown spots pretty much starting at tip of plant and making its way to bottom-old foliage 1st, younger next.
This is very advanced stage and I think based on experience that your buds will stop being compact and start producing buds/flowers with no leaves and look somewhat abnormal. I posted on this 2-3 months ago.
the good news is studies have shown that stressing the plant can result in higher THC levels, and it turned out to be some pretty strong stuff, just going to look weird. harvesting at the right time will be crucial.
too much ppms, not enough res change, salt build up, burn from excellarated respiration leading to hot spots in the leaves-freckling-and burning. excessive temps accelrate the burning
4 weeks into it, forget the nutes for now and let her cleanse her system. or maybe just the basics at 1/4 strength. you will need some silicone to help rebuild her cells. pick up the stuff Rolla8 mentioned.raise the lights as well by about 1/2 foot or more for now.