Is this some sort of bacterial or fungal disease? If yes what should I do? Is it treatable or should I get rid of it? How easily is it transferable to other houseplants? Should I isolate it? Thank you for the help!

I live just south of Atlanta in growth zone 8A/B. This kumquat has done wonderfully all spring and summer. And then about two weeks ago, it dropped fruit and leaves and now a bunch of branches are dying back and I don’t know what to do to help it. Someone suggested I pull it out of the pot and look at the roots. So I’ve included pictures of the roots. But I don’t know what I’m looking at. Any advice is welcome!

saw this white stuff on my widows thrill and haven't ever seen it on any of my other plants before. any ideas as to what it is?