Curse will take into account all the stats and passives active when it explodes except for penetration.
Not tested lately, but I'm reasonably sure it uses your stats at the time it explodes (ie mag, spell power) but takes the trait from the weapon equipped at the time it was cast (I think that includes weapon passives too like twin blade n blunt).
Not 100% sure though.
I'm back at home and tested a bit.
Curse takes 100% damage and passives from the bar when it explodes.
It inflicted exactly the same damage when cast from my resto and switched to inferno, as it did when directly casted from inferno.
The same was true for traits. Putting my nirnhoned inferno on the back and infused one on the front, the damage was always the same on front explosion, regardless of cast bar.
I'm back at home and tested a bit.
Curse takes 100% damage and passives from the bar when it explodes.
It inflicted exactly the same damage when cast from my resto and switched to inferno, as it did when directly casted from inferno.
The same was true for traits. Putting my nirnhoned inferno on the back and infused one on the front, the damage was always the same on front explosion, regardless of cast bar.