Dood... you gotta transplant into a bigger cup. The plant is too big.
That is calcium deficiency. The soil is running out of lime! Transplant!
Looks like you would be best in about a 6-8" pot (around a half gallon size), full of FRESH soil, and then start feeding with a complete grow fert with micronutirents. Also if you have some calmag handy that would be helpful, but the fresh soil WILL provide lime.