It DOES affect summoning of both elementals and nature spirits in the same way as spellcasting, as Conjuring is one of the affected Magical tests.
+1 Target Number Modifier per level of BC for the Conjuring Success test, and +1 Drain Target Number per every 2 levels of BC.
I do not think that elementals and spirits are corrupted by Background Count, although they probably don't like it. However, they would have difficulty doing things like assensing, noticing things, spellcasting (if they can use it), or using some innate abilities, just as a Mage or Shaman would in their position. The same modifiers apply to Force tests that require Magic tests (Like contesting a Banishing Conjuring test). Astrally tracking people is harder in a thick emotional haze.
Mana Warps, which are Much, Much stronger areas of Background count (BC levels 6-10) can corrupt, kill, or cause insanity in Spirits just as it can in metahumans.