Rust the scourge of the tropics. It eats away at motor vehicles, house roofs and tin or iron that is lying around.
In the garden however, several fungi are commonly called rust: most of them of the species Puccinia. Some survive from season to season on crop residues or decomposing organic matter. Others are transmitted from the seed of infected plants but more often rust spores arrive in the wind and quickly infect vulnerable or sickly plants.
In general, plants affected by common leaf rust P. hordei show circular brown pustules often with a yellow halo. Sometimes these pustules appear in clusters on leaves. Most of the leaf rusts spread very quickly during wet, humid weather, palms, day-lilies and gerberas are particularly affected.
The pustules of tropical rust P. polysora are much smaller than those of common rust P. hordei, and are produced on midribs and leaf sheaths. Most pustules are elongated or irregular in shape. Severely affected leaves die prematurely and plants are defoliated progressively from the base.
Another rust P. carthami appears on fruit and vegetable seedlings like capsicun and tomatoes and is often carried with the seed. Seedling rust appears as reddish-brown pustules at the base of germinating stems and can cause the plant to suddenly collapse.
Dark pink frangipani
Crown rust P. coronata forms round-to-slightly elongated orange pustules that are conspicuously raised. During wet or dewy weather the fungi builds up rapidly by producing large numbers of orange pustules on the leaf and leaf sheaths.
In contrast to the Puccinia rusts, the frangipani rust Coleosporium plumeriae arrived in Australia about twenty years ago and has spread throughout most of Queensland. Bright yellow pustules infect the underside of leaf surfaces, with mature fungi spores often carried by the wind. The frangipani rust is host specific to the genus Plumeria. However, there are species of Plumeria like ‘pudica’ that have resistence to frangipani rust.
Myrtle rust is a serious fungal disease that affects plants in the Myrtaceae family, such as lilly pilly, eugenia, tea tree, bottle brush and eucalyptus. Myrtle rust is widely spread in South East Queensland and recent detections have been confirmed in nurseries in Cairns and Townsville. Myrtle rust cannot be eradicated and will continue to spread because it produces thousands of spores that are easily spread by wind, human activity and animals.
Fungi spores prefer host plants that are old or weak or those with acidic residues on their leaves, which can be caused from poor nutrition or chemical-based fertilisers. In this event, with little or no resistance from the plant, the spores can easily and quickly replicate as they invade the degrading host.
Dwarf spoon-leaf frangipani which is resistant to frangipani-rust
However, most local native plants have developed a resistance to particular fungi and these plant lines should be selected for growing in the Wet Tropics. Other plants like roses and kangaroo paw are susceptible to rust infection and will always be a challenge to grow and need constant care.
By good housekeeping in the garden such as removing weeds, raking up old leaves and pruning old branches, it is easy to create unfavourable conditions for fungi. Increasing air-flow around the plant will help to reduce moist, humid micro-conditions and potential infection. Regular applications of fish-oil spray on the foliage and stems of plants, which leave a protective alkaline coating, will help the plant resist fungal infestation.
Sulphur or copper sprays can also be used to control rust. However, avoid spraying copper on bromeliads, capsicum and tomatoes as these plants are highly sensitive to copper. When spraying frangipani or ferns, do not add oil to the spray solution as this will damage and cause leaf drop of the foliage of these plants.
Fungi recycle life on planet earth and without these tiny life forms, matter would not decompose and we would soon be engulfed under megatons of waste.