Question: What is a genus of North American plants in the aster tribe within the daisy family, Sorbus or Townsendia ?

Context paragraphs:
Xanthocephalum: Xanthocephalum is a genus of North American plants in the aster tribe within the sunflower family.

Rigiopappus: Rigiopappus is a genus of North American plants in the aster tribe within the sunflower family.  It is closely related to genus "Pentachaeta".

Townsendia: Townsendia is a genus of North American plants in the aster tribe within the daisy family.

Sorbus: Sorbus is a genus of about 100–200 species of trees and shrubs in the rose family Rosaceae.  Species of "Sorbus" ("s.l.") are commonly known as whitebeam, rowan, service tree, and mountain-ash.  The exact number of species is disputed depending on the circumscription of the genus, and also due to the number of apomictic microspecies, which some treat as distinct species, but others group in a smaller number of variable species.  Recent treatments treat "Sorbus" in a narrower sense to include only the pinnate leaved species of subgenus "Sorbus", raising several of the other subgenera to generic rank.

Pityopsis: Pityopsis is a genus of North American plants in the aster tribe within the sunflower family.  Species of "Pityopsis" are known by the common names silkgrass or golden asters or grass-leaved goldenasters .

Petradoria: Petradoria is a genus of North American subshrubs in the aster tribe within the daisy family.

Psilactis: Psilactis is a genus of North American and South American plants in the aster tribe within the sunflower family which are known by the common name tansyaster.

Toiyabea: Toiyabea is a genus of North American plants in the aster tribe within the daisy family.

Triniteurybia: Triniteurybia is a genus of North American plants in the aster tribe within the sunflower family.

Sericocarpus: Sericocarpus is a genus of North American plants in the aster tribe within the sunflower family.  Whitetop aster is a common name for the genus.

Supporting paragraphs: Townsendia Sorbus
Answer: Townsendia