Welcome Lew.

Don't know how well Strelitzia grows down your way but they are normally very tough and are supposed to handle cooler conditions. I could only suggest that maybe they are too wet? They like very good drainage, prefer dryish conditions and full sun.

I don't know if this helps. Someone else is certain to know.

Hi Lew, i agree with goldhills it looks a fungus growth that would be caused with poor drainage, you can get a spray which is the same one you use on kangaroo paws which suffer from black spots.
Good luck.