Hey fennimore, I was rolling thru your thread and I really agree you need to up your nutes. That yellowing is very premature. I dont neccesarily think your plant is going to die* or all your leaves are going to fall off but I think your plant can take (and needs) much more than your feeding it. It does look good the hairs are reaching real well towards the light and the resin production sets in well but I say up the nutes and probubly in a hurry if you want to stop from stressing this baby out.. How much are you feeding?
At 3 - 4 weeks my under the canopy leaves (not the whole plant) dont have any spotting or rust discoloration, they are just yellow (if i wait long enough they turn brown) You can tell the plant just rejects / stop using it, it gets bright yellow, sags and eventually falls off by its own after shriveling up. These will always be the ones on the bottom and the ones receiving no light. This would be normal for a plant this age. This is not the problem you are experiencing. Yours, are on leaves that are receiving light And, frankley your nowhere near the end of harvest, Ive never seen a plant that was that yellow and "normal".. You have 35 more days minimum, (if your on day twenty one usually 60 - 63 days for most ive seen)....
Again, I hope you read this in full, Im not nearly ripping on you or anyone else but I do think (to a slight degree) that since you put respond and + rep in your thread title your getting people to not hardly look at the details of your grow and just say nice... You know? People that take the time to help deserve + rep bro.
But hey its been said people have different opinions.