Looks like a bird took a dump on the one leaf, or something splashed on it and that is the residue.

Looks to me like nutrient burn, or possible leaf-edge irritation.
When two or more leaves rub against each other, they can do damage to each other, and can look like that. And since the rest of the leaves in the other shots don't show this symptom, doubtful it's from applying nutrients, unless they were splashed-on and left to burn. (localized to the group that was splashed)

Also, looks like a slight ph issue.