Lynn, Evan,
Thanks for your suggestions.
I don't think it's rot caused by build-up of dead leaves. None of them had excessive dead leaves.
It may be these semps are genetically prone to rot. I'm surprised that it only seems to affect the largest, central rosette.
The issue of grubs is an interesting one. I don't have any pictures of tops and bottoms. The next time I find rot, I'll take some. I did look at them closely. Only one of the semps ever had evidence of grubs. It had small white larvae that were about 1/8 inch long. The other semps had no signs of infestation.
We did have many adult crane flies last fall. Do they have white larvae? I thought they were larger and brown. And are they that small?
I'll have to pay more attention if/when it happens again.
John
