That is a Magnesium and possible Potassium deficiency caused by unacceptably high PPM levels in the media. Generally referred to as Over Feeding or Nutrient Toxicity. It has many causes, and it is usually a slow process that snowballs over time into an avalanche. This is something I have first hand experience both failing at, and resolving.
Take a look at these pictures from my own garden last year.
I was resistant to the voices that told me it was over feeding. I thought, for sure, deficiency. I dosed with more and more magnesium supplement and didn't get any results...
Truth be told I lost a lot of this crop due to my inability to test the run off; now I understand just how important good monitoring of pH AND ppm is.
Please see my thread
The Answer to your Common Problem...
I have already answered this for everyone to see