I found this symptoms on lettuce leaves both young and old ones, are starting to become necrotic with a grey/brownish color, as if they've been burned and turned to ashes. The whole area have this symptom. I think this could be some kind of toxicity (maybe ammonia?) but not sure. It is causing serious loses to the farmer.
Here is another one starting the symptom in the border of some leaves in the low and right part of the image.
This is one of the most damaged leaves, it looks like a symptom of toxicity.