An irrigation sprinkler has a vertical axis (A) and is configured to emit a liquid jet. The sprinkler includes a base and a head operatively connected to the. A reciprocal element rotatably mounted to the head is configured to undergo reciprocal motion about the vertical axis (A), in response to being impacted with the liquid jet from a nozzle belonging to the head. The reciprocal element has a diffuser arm which carries a plurality of integrally formed, spaced apart diffuser posts which are configured to be hit by, and diffuse the liquid jet during the reciprocal motion.