Hey, i was think maybe a p or more a k def which has led to some form of infection, simply as if it was ph or over/underfert,it would be brown edges which are dry,Quick q, are the edges dry and brittle or soft and still attached?
Maybe also your night temps could be dippin to far and thats why the edges have gone black instead of brown, i had leaves turn purple from a temp of below 10 degrees c night time temps....
Sorry not much help,, maybe help from the bump