DISCLAIMER... without pics or other info I can only make the best guess. There are a couple things that can cause striping, but this is my first choice:

I'm pretty sure you have a Magnesium deficiency.
Make up a solution of 1/2 teaspoon Epsom salts and 1 drop dish soap in 1 gallon of lukewarm water.
Apply as a foliar feed. This can be repeated several times. The dead patch won't heal, but the problem will stop getting worse.
You are outdoors, right? If there is ANY way to test the pH of your soil, it's worht checking.
If the pH is where it should be, then you have a plain ol' deficiency, not a lockout, and it will be easier to fix. You can add 1/4 teaspoon per gallon of epsom salts to the next watering if that is the case.

If you have been fertilizing heavily, especially with liquid ferts, or if you are in peat moss, you may have a different problem entirely.