The perennial shrub Chrysanthemoides monilifera ssp. monilifera (Asteraceae: Calendulae), indigenous to South Africa, has become a serious weed of native vegetation in cool-temperate, winter-rainfall areas in southeastern Australia. The microcyclic, autoecious, rust fungus Endophyllum osteospermi induces the production of witches’ brooms on C. monilifera ssp. monilifera. Field observations indicate that the presence of witches’ brooms may reduce the growth and reproduction of the remaining symptomless parts of the host bushes and, therefore, this rust fungus is currently being considered as a biocontrol agent for use against C. monilifera ssp. monilifera in Australia. The vegetative growth and reproductive output of symptomless branches on bushes with different levels of E. osteospermi infections were measured at three South African sites with naturally infected populations of C. monilifera ssp. monilifera. The growth of symptomless branches on infected bushes was 26–81% less than that of symptomless branches on uninfected bushes. The number of buds, flowering capitulae, fruiting capitulae and drupes on individual symptomless branches of infected bushes was 35–75%, 45–90%, 15–99%, and 15–90% less, respectively, than those on uninfected bushes. The presence of witches’ brooms was consistently associated with reduced growth and reproductive output of bushes. Very little flowering and drupe production occurred on witches’ brooms themselves.