Crabapple bush infested by Cedar Apple Rust
No there is no benefit this late in the season. You mike look on any cedar trees you have around in the late winter and early spring to see if you can find the galls on the cedars. At this point in time the disease on the crabapple is infecting the cedar trees and then in the spring the cedars will infect the apples.
I attach a couple pictures of the early galls and when they are releasing spore with gelatinous spore horns
We have no cedar trees around. However, I will search and see if I can see any indication of any other trees being affected. If I do find any galls now or later, should I be treating the tree/bush?
Look at this article to see symptoms on apple and cedar.
Probably not worth the time to spray the cedars.