The rusts are amongst the most common fungal diseases of garden plants. Trees, shrubs, herbaceous and bedding plants, grasses, bulbs, fruit and vegetables can all be affected. Rust diseases are unsightly and often (but not always) reduce plant vigour. In extreme cases, rust infection can even kill the plant.
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.
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
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, leafy 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
Edibles and Ornamentals: Difenoconazole (liquid concentrate only) (Westland Plant Rescue Fungus Control) may be used to control rust diseases on fruits and vegetables and copper-based fungicides (Vitax Bordeaux Mixture, Bayer Garden Fruit and Vegetable Disease Control) may be used to control rust on currents and gooseberries.
Ornamentals only: The fungicides difenoconazole (ready-to-use spray) (Westland Plant Rescue Fungus Control), myclobutanil (Bayer Garden Systhane Fungus Fighter and other formulations), tebuconazole (Bayer Garden Multirose Concentrate 2) and triticonazole (Scotts Fungus Clear Ultra, Roseclear Ultra) are approved for the control of rust diseases of ornamental plants.
Some formulations also contain insecticides but these are best avoided if no insect pest problem is specifically identified. For example, some formulations of myclobutanil (Westland Rose Rescue) contain cypermethrin, some formulations of tebuconazole (Bayer Garden Multirose Concentrate 2) contain deltamethrin and some formulations of triticonazole (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun) contain acetamiprid.
The foliage and flowers of some ornamentals are sensitive to damage by certain fungicides, particularly some formulations of difenoconazole and myclobutanil. Check the product label before use.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
Most of the Wisley Advisory Service enquiries regarding rust occur on the following hosts: allium species, antirrhinum, bluebell, box, chrysanthemum, fuchsia, 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.
Common name Rust diseases
Scientific name Various
Plants affected Many different garden plants
Main symptoms Pustules on leaves and occasionally on other aerial parts. Pustules may be orange, yellow, brown, black or white in colour
Caused by Fungus
Timing Seen mainly in mid- to late summer and autumn