By the looks of it, seems like you have a potassium problem. Next time you fertilize, try using something with a higher 3rd number, like 10-10-30. The 3 numbers stand for N-P-K, Nitrogen-Phosphorus-Potassium. According to my guide book (Agriculture Text book), yellowing and browning of leaves with enlarged veins are signs of (K+) deficiency.

Maulrat