I have a deshojo, a newly airlayered palmatum, and an established palmatum
all in the garage (about 5 degrees C) and have noticed many of the longer
branches are blackening on all three trees.

I don't know about the others, but 'Deshojo' is notoriously difficult. I gave
up after I lost two expensive ones, possibly to disease.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)