In late summer and fall, garden asters are often infected with rust disease. Severe leaf rust usually leads to death of aster leaves and reduced plant vigor. There are several species of rust fungi that occur; common aster rust is caused by the fungus Coleosporium asterum. Most rust fungi spend part of their life cycle on more than one host, often completely different hosts. The alternate hosts for this aster rust fungus are two- and three-needle pines which, when infected, show needle rust symptoms. One of the possible but often impractical means of controlling the rust is removal of the alternate host from the surrounding area (i.e., removing pines). The remaining option is to spray the asters with a fungicide to protect the plants from infection by the rust fungus. There are several fungicides available that will control rust diseases. Check the fungicide label for spray rates and timing to manage aster rust. Contact your local Extension office for fungicide recommendations.
For photos of the disease on both pine and aster, visit: Coleosporium Needle Rust of Pine (Coleosporium asterum)