Myrtle rust

Myrtle rust is a disease caused by the fungus Austropuccinia psidii (formerly Puccinia psidii).

Symptoms

• Myrtle rust causes brown to grey spots, often with a red-purple halo on leaves.
• Leaf spots can be visible on both leaf surfaces.
• A few days after infection, masses of bright yellow or orange-yellow spores (powdery specks) appear on the spot's surface.
• Symptoms can be variable, dependent on host species and infection phase.

Plant stage and plant parts affected

• Spots, and subsequent spores, can form on new leaves, shoots, fruits and flowers.
• New leaves and shoots are more vulnerable.
• The infection damages plant parts causing deformation, defoliation, dieback and stunted growth.
• Infection of flowers and fruit can reduce the ability of the plant to reproduce.
• Highly susceptible plants may die following periods of repeated infection. The time it takes for plants to die depends on species, size and environmental conditions.

Different types of rust disease can be found on a variety of plant species, however myrtle rust only occurs on plants in the Myrtaceae family.

• Widespread across Queensland.
• Mainly along coastal regions east of the Great Dividing Range; especially in areas with mild temperatures and high humidity or high levels of leaf wetness.
• Limited detections have been made west of the Great Dividing Range.
• Not yet detected in Western Australia.

Around 400 Australian species from more than 50 different genera within the Myrtaceae family.

Plants affected include gum trees (Angophora, Eucalyptus, Corymbia), lilly pilly (Syzygium, Acmena, Waterhousea), bottle brush and tea tree (Leptospermum, Melaleuca), brush box (Lophostemon), myrtle (Backhousia), turpentine and satinay (Syncarpia), Lenwebbia, Rhodamnia, Rhodomyrtus, Austromyrtus, and guava (Psidium).

The life cycle of myrtle rust in Australia involves the production of 2 types of asexual spores.

Asexual spores

• Bright yellow to orange-yellow.
• Produced quickly, in very large numbers and spreads the disease to new hosts by wind, animal and people movement.
• Can survive for weeks in suitable conditions.
• Warm, sunny days with heavy morning dews provide perfect conditions for myrtle rust infection and spread. High humidity and moderate to mild temperatures favour spore germination.

Sexual spores

• Darker, orange-brown spores.
• These spores are commonly seen in Australia, although they are reported as rare in Brazil. The factors that influence their occurrence are not well understood but can include stage of disease, decline of the host, leaf age and temperature.
• Myrtle rust spores need surface water to germinate, and optimum conditions for infection include low light or darkness and mild to moderate temperatures.
• Myrtle rust symptoms vary based on host, leaf age and stage of infection.

• Some plant species are highly susceptible and are killed by the disease.
• Decline of species – once common species are now listed as Critically Endangered, for example: Rhodamnia rubescens, Rhodomyrtus psidioides.
• Some plant species struggle to reproduce or compete with non-Myrtaceae species or those less susceptible to the disease.
• The disease has a significant impact on some species, with flow on impacts to Queensland's biodiversity and commercial industries that use myrtaceous plants, such as cut flower, nursery, garden, native forest timber, lemon myrtle, tea tree oil and bee keeping businesses.

• Large numbers of spores are easily spread by wind, human activity and animals.
• Movement of infected plant material can spread the disease over long distances.
• Disturbances such as fires or storms, and the subsequent flush of regrowth, can provide ideal conditions for infection.

Monitor Myrtaceae plants for signs of myrtle rust.

If detected, take action to prevent the spread of the disease.

It is important to understand the impacts that this disease will have in the short and long term. It is also important to look at plants and trees that might be free of the disease as these may be able to be used in the conservation of species.

Protect your farm from plant pests and diseases:

• Read the Biosecurity manual for the nursery production industry (PDF, 3.2MB)
• Read the Biosecurity manual for the plantation timber industry (PDF, 1.9MB)
• Protect your farm from emergency plant pests
• Understand good farm biosecurity
• Visit the farm biosecurity website

A number of fungicides are registered for the control of myrtle rust. Consult your local garden centre for a list of approved products.

If you think you have found myrtle rust, you must take all reasonable and practical steps under your control to minimise any associated risks, including limiting further spread. This is called a general biosecurity obligation (GBO).