Rusts are ugly, but interesting diseases, the plural is used here because there are many, many different organisms that produce rust disease. Rust is often host specific and affects some of our favorite ornamentals such as geranium, snapdragon, fuschia, aster, chrysanthemum (both brown and white rust), daylily, heuchera, veronica, shrubs and trees like roses and hawthorne, conifers as well as small fruits like raspberries. Monocots like cereal crops and grasses are suspectible to rust as well.
Rusts are classified by their life cycle on host plants. Simple rusts only need one host to complete the life cycle while complex rusts require an alternate host, unrelated to the plant which show symptoms when infected. Similiar to powdery mildew, rust is a obligate parasite, meaning a host plant is needed for the organism to grow and reproduce. The spores germinate and directly penetrate epidermal leaf cells or enter through the stomata depending on the rust species.
Rust outbreaks are triggered by high humidity when the pathogen is present. Five hours of leaf wetness at temperatures ranging from 58-77o, depending on the rust species, provides conducive conditions for germination. Basidiospores germinate and directly penetrate epidermal leaf cells or enter through the stomata depending on the rust species.
Early signs of rust are most often faint, yellow leaf spots on the upper leaf surface. This early spotting can be easily mistaken for other disorders. The pustules, or masses of spores, that then form will confirm a rust disease is present. Pustules can be formed on both upper or lower leaf surface as well as on the stem, but are most common on the leaf underside. Depending on genus, the pustules may be very colorful, ranging from yellow, brown, or orange to a buff white color. Rust is spread by wind-borne spores and through plant shipments. Plants may be infected without showing visible signs for weeks to months. Spores can survive for a few months without association with a living host.
Cultural management options are very similar to powdery mildew, although avoiding extended leaf wetness is crucial with rust diseases. Avoid watering late in the afternoon so that foliage can dry before evening. Quarantine incoming plants known to be suspectible and use preventative spray treatments at 14-21 day intervals. Make note of any suspectible plant cultivars being grown at your facility. Use this list to target scouting for early symptoms. Fall asters and chrysanthemums are both particularly susceptible to rust.
With rust, once an infection occurs, plants quickly become unsalable. A preventative fungicide program should be started in spring to early summer on outdoor crops, with applications every 14 days. Broad spectrum fungicides Daconil or Concert II (outside only) provide a good basis for a preventative spray program. Rotate with another MOA such as one of the strobulurin products, Heritage (MOA 11) or Broadform, Mural, Orkesta, or Pageant (all MOA 7 + 11). Due to the multiple modes of action of each of the products above, these two classes can provide a valid chemical rotation. Once plants are in bloom, do not use Daconil or Concert II to prevent flower damage.
Two NEW products offering effective for rust prevention are Avelyo (MOA 3) and Postiva (MOA 3 + 7). Both share MOA 3 with Eagle 20EW, which is regarded as the best curative product we have for several rusts. For this reason we recommend they not be used as the basis of a rotation program, but rather to extend the rotation as needed.
Disclaimer: Not all products are registered in all states. Some pesticides are restricted use in some states or regions and not others. It is the responsibility of the applicator to read and follow all label directions, remembering that labels may change. Other products may be safe and effective. Rates, application methods and edible status are detailed in our GGSPro Insecticide & Fungicide Guides.
