I think I know what this is... but i don't want to say anything cuz I want people's opinions without being influenced by what I think. The white build-up on the edges is from the wrong kind of soap being used as a wetting agent for a neem application. It has since been rectified.
Interesting, are you thinking Potassium deficiency? I found this in the help section, and it looks like Picture D is almost identical to your situation. Unfortunately they are not very clear on the difference between a deficiency and toxicity.