Sphaerothecum destruens (the rosette agent) is a parasite of fish.

[1][2] It was first discovered in the United States in association with invasive species including topmouth gudgeon, but was found to be the causative agent of a disease in the UK affecting salmonid species such as Atlantic salmon and brown trout.

It is thought to pose more of a risk in Europe than in the US, as native species there are more susceptible to the parasite.

The disease causes high rates of morbidity and mortality in a number of different salmonid species and can also infect other UK freshwater fish such as bream, carp, and roach.

[3] The genus Sphaerothecum is closely related to the genera Dermocystidium and Rhinosporidium.