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Need to know:
- Affected leaves will have raised brown spots on undersides of leaves.
- Wet conditions promote infection because disease is transmitted by wind and splashing rain.
- Removal of stalks, stems, and leaves is necessary to treat the infected plants.
- Fungicides can be used for disease management.
Overview of hollyhock rust
Hollyhocks are grown for the their showy, colorful flowers. A common disease problem on hollyhocks, called rust, can also lead to 'colorful' foliage.
Symptoms of hollyhock rust
Small orange to yellow spots form on the underside of infected leaves. If the stem of the plant is infected brown irregular areas may form as well. Severely infected leaves may develop holes and die. The raised spots may coalesce, killing larger areas of leaves. Orange pustules also form on the stems of the plants.
Signs of hollyhock rust
The rust fungus causes raised brown spots, usually pinhead-size or larger, on the undersides of leaves. Eventually they burst open releasing spores of the fungus.
Disease cycle of hollyhock rust
Puccinia malvacearum, the rust fungus that infects hollyhock, causes yellow spots on the upper leaf surface, and orange-brown raised pustules on the lower leaf surface. Wet conditions promote infection by the rust fungus. The lower leaves typically show symptoms first, and the disease slowly progresses to upper leaves over the summer. Infected leaves eventually turn brown, wilt, and die. Wind and splashing rain help spread the spores of the fungus, so spacing plants to promote good air circulation can help slow the progression of the disease. Because wet conditions favor infection, water the soil around the plants rather than wetting the leaves with overhead irrigation if possible.
Type of Sample Needed for Diagnosis and Confirmation
The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if you plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states diagnostic laboratory for U.S. residents. If your sample is from outside of Iowa please do not submit it to the Plant & Insect Diagnostic Clinic without contacting us
Management of hollyhock rust
Because the rust fungus survives from season to season on infected plants parts, cut back the plant stalks at the end of the season and thoroughly remove all of the stalks, stems, and leaves. The hollyhock rust fungus can also infect the common mallow weed, so weed control in the vicinity is important.
In most situations, these good gardening practices alone will not control rust completely. Fungicide applications can be used for additional disease management. Fungicides should be applied when symptoms are first observed and reapplied at approximately 7-10 day intervals. Be sure to read and follow the instructions on the label of the fungicide. You can find fungicide products at your local garden center.
Fungicide applications may be avoided by following good Integrated Pest Management practices like those listed in this encyclopedia article. Often, the only preventative application is effective to manage plant diseases. If the problem requires a fungicide, state law requires the user to read and follow all labels accordingly. For more information, read Proper fungicide use.
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