This is an interesting discussion about mulberries in Florida. I bought one many years ago at Home Depot labeled as M. nigra and it is almost bulletproof. I had always thought that one of the defining features of M. nigra was that it didn't have seeds. I have never seen seeds in mine (although I have noticed tiny seeds in the Pakistani mulberry that I obtained from Pine Island Nursery many years later). I believe that Pakistani is a M. alba cultivar (which to my understanding does have seeds). I wonder if nematodes are the issue instead of humidity?
I understand Florida has a different type of RKN than we have in San Diego. My mulberry tree is planted in an RKN infested area, but shows no issues above ground.
The defining phenotypes of M.nigra are black buds when dormant, very juicy berries with high acid, poor performance outside of dry climates.
If pollinated with a male M.nigra they produce fertile seed, mine have a crunch to them, but never produce viable seed.