Then again, like archer said what is the pH? All readings that you can get. Water/feed pH? Run off pH? But most especially soil pH.
gert yourself an Accurate 8 soil probe.those seem to be some of the most accurate and longer lasting probes that'll give you a decnt read as to how you Soil PH is.I dont use them anymore but Im organic.nutes or organics man gots ta have one imo.ive seen them for 40-60 bucks,and if ya take care of it it'll last ya a good bit.a digital PH and TDS meter and that's all you should need imo.
and if you having soil PH issues you can do one of several things other than ripping lower PH water through it every time which is a hassel in my eyes.but turning already broken down lime in a soil as a top coating works pretty well,Hydrated Lime is fast and effective but very potent and I typically dont like to tell ppl to get that cuz it can screw up a grow pretty quick if ya dont know whatcha doin or get a lil carried away as new growers(myself included in the day too)can easily get flustered.but turn lime into your topcoat and fluff it into the soil and keep it moist and two weeks(thats why I break some down prior to me growing right into a soil coating) later it'll buffer it well. but there are a few options avaliable and if your chems or organics much of this will work well imo.hope this help ya man and good luck bud!
Looks to me like ya either splashed your leaf but unlikely,I'm thinking cal mag imo. Have ya treated her lately?

If you're not seeing any traces of pests I'm really inclined to believe it is pH related. If you were locked out your lower leaves will yellow and die as they give up their reserves while the top stays green. It's a bummer that you're having so many issues.

Top's all browned sugar leaves. In fact at this later stage there's still some life left on the bottom, all crisp up top.
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Thanks for the sympathetic nod.
Would think with the three pix in this thread, early, mid, and later it might start becoming clear. No body knows.