Rusts
General Recommendations: Rusts are a highly specialized group of fungal pathogens of higher plants and of some ferns. They are obligate parasites, meaning that they are dependent upon a living host to complete their life cycle certain spores produced, though, can survive on living or dead tissue and can start the disease cycle in the spring. This specialization is also expressed in the fact that rust fungi have narrow host ranges and usually infect only one type of plant. This means that if a rust-infected plant grows close to another healthy and unrelated plant the second plant will be unlikely to get the rust disease. However, because large amount of spore inoculum are able to travel long distance by wind (the mechanism of rust spore dispersal) closely related plants growing close to each other are likely to become rust-infected.

With the exception of cedar apple rust and cedar quince rust, these diseases are not very common in ornamental plantings and overall do little harm. Rusts get their name from the color of the spores generated in the repeating part of its life cycle. Symptoms may include leaf yellowing, withering and early leaf drop. In some cases at the time orange spores of the fungus are produced, lesions are not yet evident.

Most rust diseases depend on two different hosts to complete their life cycle. Rust disease control thus entails interrupting the life cycle on at least one of the hosts by planting resistant varieties, removing host plants, cleaning up of diseased debris and/or managing disease with fungicide. Removing one of the alternating hosts may not be feasible unless it can be done for several miles around the site. Therefore, most strategies for controlling rust diseases employ the use of fungicides and the selection of resistant cultivars.

 Control of Rust Diseases:

1. Follow good sanitation practices. At the end of the season, remove all diseased debris and bury or discard away from the garden. Since leaves, stems and flower stalks can harbor the fungus over the winter, cut down all above ground parts. In the fall, pick up and remove any fallen leaves.

2. Check the area for any related weeds or ornamental hosts that might harbor the rust fungus over the winter. If propagating from stock plants, make sure they are not infected. Discard stock plants every third year. Keep stock plants in a greenhouse and never outside.

3. Removal. As soon as rust pustules are found, the infested plant part should be removed.

4. Apply a preventitive fungicide. Rust diseases can be managed by timely applications of a fungicide labeled for control of rust diseases starting when new growth begins in the spring and continuing for 5 or 6 applications every 7 to 10 days.

5. Plant a resistant cultivar. For certain rust diseases like cedar apple rust, resistant apple and crabapple cultivars are available. The same is true for cedar hawthorn rust, but not for hollyhock rust. Geranium rust is most prevalent on zonal types including the common florist’s geranium propagated by cuttings and some seed geraniums. Resistant types include the ivy, regal, scented and wild type geraniums.

Other images

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Pustules of rust are visible on the underside of a leaf of this goldenrod (Solidago) but it is also infested with midges that caused the large leafy rosettes called goldenrod bunch galls
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Unknown rust on leaves and on a rosette gall caused by the midge Rhopalomyia solidagninis on Canada goldenrod (Solidago altissima)
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Yellow pustules of an unidentified rust on the underside of goldenrod leaf (Solidago)
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The most common symptom of rust on iris (Iris) is yellowing and browning leaves; closer observation will reveal rust-colored spots
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Close-up of rust pustules on iris (Iris)
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Rust on a cottonwood leaf (Populus deltoides)
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Rust on jack-in-the-pulpit leaves (Arisaema triphyllum)
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Rust pustules on underside of jack-in-the-pulpit leaves (Arisaema triphyllum)
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Close-up rusty colored pustules of rust on blackberry (Rubus)
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Rust on morning glory (Ipomoea)