Rusts Information
Causal Agent(s) and Hosts
Rust is a common disease of many ornamental plants. There are several fungal genera that cause rust, with Puccinia, Uromyces, Phragimidium, and Pucciniastrum being the most common. Rust fungi have narrow host ranges. Some rusts cycle between alternate hosts while the primary host is dormant or absent, such as cedar-apple rust.
Symptoms & Signs
Rust fungi are obligate parasites, meaning they require a living host to survive. As a result, rust fungi do not typically kill their host. Plant symptoms include yellow or orange spots on leaves, and severe infections can cause who leaves to become chlorotic or necrotic. Whole plants may become stunted and wilt-prone under extreme levels of infection by rust fungi.
Rust gets its name from the small, raised circular spots, or pustules, that form on leaves, stems, and petioles. These pustules house spores that are yellow-orange to brown in color. Gently rubbing the pustules with your hand or a tissue will dislodge the spores.
When Does it Occur?
The life cycle of rust fungi varies greatly. Some rusts complete their entire life cycle on a single host, while others require two or more hosts to complete their life cycle. Spores are moved short distances by wind, water, insects, and animals. Wind can also move spores across large distances. In general rusts will overwinter as spores or mycelia in plants or plant debris. Spore germination requires water on the plant surface. Rust fungi are most problematic during extended periods of moderate temperatures.
Cultural Management
Plant resistant varieties, if available. Avoid overhead irrigation where possible. If overhead irrigation is unavoidable, irrigate early in the morning to avoid prolonging leaf wetness periods. Maintain adequate spacing between plants to avoid restricting airflow. In perennial plants you can prune out infected material. Infected plants should be removed and destroyed in greenhouse or nursery settings to protect uninfected plants. For rusts where the alternate host is known, destruction of the alternate host can disrupt the disease cycle and reduce the disease.
Chemical Management
There are many fungicides available to manage rust fungi. Fungicides should be used preventatively for best results. Some fungicides have varying efficacy on different rust fungi, and other fungicides are only effective against specific rusts. Always check the label to ensure a selected product is effective in your situation.
Selected fungicides for rust management
1 Not all commercially available trade names are included to save space. Some products may have use restrictions. Always follow label directions for rates and use sites. Mention of specific products does not constitute endorsement.
2 P = Preventative; C = Curative
3 REI = Restricted Entry Interval
4 Relative efficacy. + = Marginally effective; ++ = Moderately effective; +++ = Effective; ++++ = Very Effective; ? = Limited information at this time.
5 Fungicide Resistance Action Committee code(s). Products sharing a code contain active ingredients with the same mode of action. Always use resistance management practices.
6 Resistance Risk: L = Low; M = Moderate; H = High, U = Unknown
