http://ctrf1.com/2013/08/harvesting-has-started-on-some-farms/ said:
We have received some calls recently in regards to the tips of leaves yellowing and some of the leaf tissue at the tip of the leaf drying up (click here for photo "below"). This is potassium (K) deficiency, which can be seen sometimes when the plant is growing rapidly at about the topping stage. This is not necessarily an indication that more K is needed. When the plant is growing rapidly, it sometimes can’t take up all the K it needs.