Rusts are common plant diseases. They mainly produce orange or yellow spots on the upper surfaces of leaves, and occasionally on stems and sometimes on flowers and fruit. Some rust diseases produce brown, black or white spots. Corresponding orange to brown pustules then appear on the lower leaf surface later in the year.
Each species of rust fungus has a narrow range of plants it will attack, which is made up of one species or a few closely-related plants. For instance, leek rust attacks ornamental alliums, onions, garlic and chives. But antirrhinum rust won’t attack roses, chrysanthemum white rust won’t attack fuchsias. This means each one doesn’t spread as far as many people think.
The rust disease tends to start on the lower/younger leaves, and then spreads further up the plant.
Heavy infections can severely reduce the vigour of the affected plant and, in some cases – especially small, young or already weakened plants – can even kill the plant.
Commonly affected plants include the following.
Pelargonium (bedding geraniums)
Rhododendron and azalea
Leek, onion, chives and garlic