BBing -

I was experiencing a very similar if not identical issue when I first started growing hydro. Even though I was using what was supposed to be a "full spectrum" nutrient line, it turned out that I was deficient in one or more minerals. I ended up figuring it was a magnesium deficiency. Botanicare makes a product called CalMag that is the shizz. I add 1 tsp. (5 ml) per gallon (4 liters) and I have not experienced the yellowing/crispy edge/black spots since.

I have a photo album with common nutrient deficiencies and what they look like in cannabis plants. I can't remember what website forum I downloaded them from, and I don't know what book they are from, but full credit goes out to whoever is responsible.

Check it:

Segregate the plants until you are sure of the problem. This way, if it's something bad, they won't infect the others. It may just be leaves that had a lot of the borg on them. Black spots says mites to me! Also, fan leaves yellowing & falling away during flowering is a regulr thing. Particularly the lower leaves. I am 7 weeks into a run & pick up a dozen or so everyday.

Thanks for the help guys.... I'll try and add some mag amp by cutting edge. I already have some on hand. We'll see if that works