Studies on the respiration of 'S. homoeocarpa' revealed that both hypovirulent and asymptomatic isolates use the alternative oxidase (AOX) pathway for ATP production and growth, but the pathway is only transcriptionally upregulated in the hypovirulent isolates. The development of stroma by 'S. homoeocarpa' was influenced by reactive oxygen species (ROS) scavengers and light, indicating that stroma formation is influenced by ROS. Hypovirulent isolates did not produce stroma. The formation of sterile apothecia, which have rarely been reported for 'S. homoeocarpa', was observed. Apothecia contained densely-packed, filiform, unbranched paraphyses, arising from a hymenial layer, but asci were not observed. The formation of apothecia was increased in the presence of ascorbic acid and was affected by light intensity. Hypovirulent isolates did not produce apothecia. The transmission of OMV3a from hypovirulent to virulent recipient isolates was efficient, and differences in the resulting morphology of converted isolates were observed. The study presented here indicates that OMV3a alters respiration, impairs development, and reduces host colonization of hypovirulent isolates of ' S. homoeocarpa'.