The yellowing doesnt seem to be concentrated in one spot or another, so I assume its not a nute issue. You really should get a cheap EC meter though. They really like N during the stretch so it could be an N deficiency, but luckily if you're already in the stretch, she won't need hardly any N soon and you can beef up on the P. Keep an eye out, if it isn't spreading then don't worry. Watch your watering habits too.