Many people refer to any disorder which causes reddish-brown spots on plants as rust. However, a true rust is caused only by a fungus. Rusts are parasites that depend on living plants for their survival. They create brown or rusty pustules, usually on leaves but sometimes on stems and flower buds.
They are related to smuts (a fungal disease of cereals) and certain mushrooms. Smuts, as the name implies, create black sooty masses on their host. They are more of a problem for the farmer than the gardener, as they mainly attack farm crops.
House plants that can become infected with rust include pelargoniums and chrysanthemums. In the greenhouse or conservatory, carnations are most at risk. Outdoor plants that can be attacked include Sweet Williams, antirrhinums, roses, hollyhocks and anemones, and herbs such as mint.
While some rusts will only attack a specific variety of plant and its close relatives, others can live on a number of plants that are totally unrelated.
Understanding rusts
An understanding of how rusts reproduce is of some help when trying to get rid of them. Hollyhock rust has a relatively simple life style.
Characteristic symptoms of rust on a leaf.
The mycelium (a mass of branching threads) growing within a leaf produces spores in small pustules, which break out on the surface. Each spore eventually germinates, becomes detached from the plant and is blown around in the air, infecting healthy leaves. When it lands on a leaf it penetrates the leaf tissue and creates mycelium and eventually forms more pustules. It is therefore necessary to break this cycle to prevent the development of the rust.
Some rusts, such as those that attack conifers, have a much longer life cycle and have two different host plants. It is much more difficult to get rid of these types of rust.
Combating rusts
Rusts are not easy to get rid of, because once they have entered a plant the fungal threads, or mycelium, usually travel throughout the tissues. To control rusts on annuals and perennials remove and burn all the infected leaves and spray the plants with a product containing thiram or mancozeb.
Indoor plants and rust
In conservatories and greenhouses, keep the humidity low around carnations and chrysanthemums, as high humidity encourages rusts. Indoors, do not mist susceptible plants too often. Remove any diseased leaves and destroy badly infected plants.
Controlling rust outdoors Vegetables and herbs that are deficient in potash are susceptible to rust attack. Correct the deficiency with a light application of sulphate of potash. Destroy any plants that are severely affected.
Small shrubs with rust-infected leaves should be sprayed with thiram or mancozeb. Infected shrubs and trees should be watered, fed and mulched regularly, as dry compost, and compost that is deficient in nutrients, encourage rusts.
Rusts on antirrhinums
Infected antirrhinums have small brown pustules; the lower surfaces on the leaves are the first to show these symptoms. The plants become stunted and unsightly, and wet weather encourages the spread of the spores. Treat as for annuals and perennials.
Roses and hollyhocks
Rose rust creates numerous small yellow spots on the undersides of the leaves during the height of summer. These darken with age to dark brown or black. If only one variety is infected, remove and burn it to safeguard the others. Always burn shoots cut off during pruning, and cut out badly infected shoots at any time. Spray with thiram or mancozeb.
Hollyhock rust first appears as orange-yellow pustules (which later turn brown) on the undersides of the leaves. Burn all badly infected plants to prevent the rust spreading.
For slight infections, spray with thiram or mancozeb. Rust resistant varieties are available, but it is best to raise from fresh seed every other year.
Plants to watch
Rusts are not a major problem with house plants, but they can attack certain container-grown plants. However, with early treatment the damage can be kept to a minimum. The following are particularly susceptible to rust. Watch them carefully; if rust appears, treat immediately.
Chrysanthemums are very susceptible to rust; keep an eye on them and treat any rust attack immediately.
Checklist
Do
- Remember to burn any fallen diseased leaves that are lying around your plants.
- Buy varieties of hollyhocks and antirrhinums labelled rust resistant. This will reduce the chances of them being attacked by rust.
Don’t
- Propagate from plants infected with rust.
- Leave pruned stems lying around your plants. Gather them up and burn them.
- Keep the atmosphere in a greenhouse or conservatory constantly moist.
- Mist susceptible plants too often.
