At a guess.....

Arr! You may be experiancing the black pear me ol matey! Visited me the other day twice! Its like death has visited you plants and left its mark. All life is drained out of that part of the plant and it turns black. I just cut mine off and binned it. Made sure the fans were hitting the plant, still got another visit 2 days later, but if u catch it early enough..... Good ventilation is all important.