Hi there: This sounds like either a deficiency in Nitrogen or Iron or both......or sun scald. Have you fertilized it lately? If it was my plant, I would fertilize it quickly with something with a high Nitrogen number. The numbers on fertilizer are N-P-K, so x-x-x or N higher than the others. You could add some chelated iron to the ground too. I've never added iron before, so I don't know if it's a powder or what, but I know you can buy it.
I hope this helps.
