What are rusts?

The rusts are a group of fungal diseases affecting the aerial parts of plants. Leaves are affected most commonly, but rust can also be found occasionally on stems and even flowers and fruit.

The spore pustules produced by rusts vary in colour, according to the rust species and the type of spore that it is producing. Some rusts have complex life-cycles, involving two different host plants and up to five types of spore.

Symptoms

You may see the following symptoms:

• Pale leaf spots eventually develop into spore-producing structures called pustules
• The pustules are found most commonly on the lower leaf surface and produce huge numbers of microscopic spores
• Pustules can be orange, yellow, brown, black or white. Some are a rusty brown colour, giving the disease its common name
• In some cases there may be dozens of pustules on a single leaf
• Severely affected leaves often turn yellow and fall prematurely
• Pustules also sometimes form on leaf stalks (petioles), stems and, rarely, on flowers and fruit
• Heavy infection often reduces the vigour of the plant. In extreme cases (e.g. with antirrhinum rust) the plant can be killed

Control

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

Non-chemical control

Detailed recommendations are given in the profiles available for rust diseases of individual plants and crops. General recommendations are as follows:

• The development of the disease can sometimes be slowed by picking off affected leaves as soon as they are seen, provided that this involves just a small number of leaves. Removing leaves in large numbers is likely to do more harm than good
• Provide conditions that encourage strong growth, but avoid an excess of nitrogen fertiliser as this results in soft, lush growth that is easily colonised by rust
• All diseased and dead material should be removed at the end of the growing season. The overwintering spores of some rusts are very resilient, so do not compost affected material
• Resistant cultivars are available for some plants and crops, but this resistance is occasionally overcome by genetic changes within the rust fungus
• In some cases no control is required. For example, many rusts of trees develop too late in the summer to have a significant effect on  vigour, even though the whole tree may appear yellow or orange in late summer due to the huge number of rust pustules on the leaves

Chemical control

No fungicides are currently available to home gardeners for the control of rust diseases on edible crops.

On ornamental plants the fungicides tebuconazole (Provanto Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect), and triticonazole (Fungus Clear Ultra) are approved for the control of rust diseases. 

The following products contain a combination of both insecticide and fungicide, enabling the control of both insect pests and disease: myclobutanil containing cypermethrin (Resolva Rose 3 in 1, Rose Shield Bug & Fungus Killer, Roseclear Ultra Gun 2, Rosegarde) and triticonazole containing acetamiprid (Roseclear Ultra, Roseclear Ultra Gun).When a proprietary product contains an insecticide as well as a fungicide it would be preferable to use an alternative product if pests are not a problem on the plants treated.

The foliage and flowers of some ornamentals are sensitive to damage by certain fungicides. Check the product label before use.

Inclusion of a product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.

Download

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Links

Chemicals: using a sprayer
Chemicals: using safely and effectively
Chemicals: storing and disposing safely

Biology

Most of the RHS Gardening Advice enquiries regarding rust occur on the following hosts: allium species, antirrhinum,  bluebell, box, chrysanthemum, fuchsia, heuchera, hollyhock, hypericum, mahonia, pear,  pelargonium, poplar, rose and vinca.

Rust fungi have very limited host ranges. For example, the rust that attacks antirrhinum is a different species from that affecting hollyhock. By late summer there may be many different rust species present on various plants in the garden.

Infection is favoured by prolonged leaf wetness, so rusts are usually diseases of wet summers.

Some rusts spend their entire life on one plant and produce just one or two types of spore. Others need two, often completely unrelated plants in order to complete their life-cycle, and can produce up to five different spore types. European pear rust, for example, spends part of its life on juniper.

The colour of the rust pustules varies according to the rust species and the type of spore that it is producing. Rose rust, for example, produces orange pustules for much of the summer, but in late summer and autumn these are replaced by black pustules containing overwintering spores.