Okay, I know Dusk Balls have the best catch rate when used at night or in a cave, but does Primal Groudon's Desolate Land weather effect remove those conditions and make their catch rate only 1x?
Harsh sunlight has no effect on Dusk Balls.
According to this Bulbapedia article, the Cave of Origin is considered a location in which Dusk Balls have a boosted catch rate. If we tell the Cave of Dragonflies' catch calculator that we're catching Primal Groudon with a Poke Ball at full health, it says we have around a 1.6% chance to catch it with any given ball. If we were to choose the Dusk Ball and click the cave checkbox instead, that chance goes up to roughly 4%.
Neither this catch calculator or Bulbapedia's article on harsh sunlight give any indication that harsh sunlight affects catch rate in any way.
No, if dusk balls aren't under darkness or in caves, then it has the same catch rate as an ultra ball
It depends what time of day it is, if its day time,the pokeball has the catch rate of a ultra ball,if its dark the duskball has a higher catch rate, Hope this helps. Diamond :D