Spring infections form minute yellow cups of spores on the undersides of rose leaves. The spores become orange in reddish orange cups by summer. These spores repeatedly reinfect the rose. By late fall, the spores are black. These black spores overwinter on fallen leaves and form microscopic, colorless spores that infect leaves in spring.
Severe infections defoliate the plants.
Apply a foliar fungicide, as needed, to control the rust. Also, rake leaves in compost or burn them. This information is for educational purposes only. References to commercial products or trade names does not imply endorsement by MSU Extension or bias against those not mentioned. This information becomes public property upon publication and may be printed verbatim with credit to MSU Extension.