I have a problem with my plumerias. All of them despite being of different species. Usually the middle part of the leaf turns black and then in a couple of days the leaf is dead. Sometimes the whole new leaf turns black. Could it be the coco coir I use ad medium (with lots of pumice and perlite and some manure)? In the past I didn't use manure but still had the same issue. I use the same coco coir for all my plants btw but they don't have any similar issues.