Circular raised blisters on Ambrosia trifida. May be caused by some kind of rust fungus, cf Albugo tragopogi or Puccinia xanthii. Observed in Texas in May.

Possibly Protomyces:

• Underside of spots looks very like Taphrina, but Taphrina is not known for infecting Asteraceae. Protomyces is related to Taphrina and causes similar symptoms.
• P. gravidus causes large stem galls on A. trifida in Ark. and La. See Cartwright RD, Templeton GE (1988) Biological limitations of Protomyces gravidus as a mycoherbicide for giant ragweed, Ambrosia trifida. Plant Disease 72:580-582. doi.org/10.1094/PD-72-0580