have exactly the same problem, thought it was too much pk, but now i think it has to do with over watering. always happens late in flower, but its not bud rot. have tried heaps of different things to help it, had a couple of successful runs but then it came back. doesn't happen to other strains I've tried so moving on even tho it was a killer smoke. guy at the store said it was some type of rot from moisture in the air, nasty stuff, will ruin your plants. did you ever find a solution? thinking about trying some good bacteria for the roots, maybe give the plant enough help to fight it off. tried lots of silica but that didn't help either. come to think about it, when i ran it with a low ec i got less of it.