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Common Tree Diseases In Australia

Myrtle Rust

Despite the Australian Government’s strict biosecurity, plenty of tree diseases have made their way into the country, some of which are devastating to our precious flora. In this article, we’ll explore some of the most common tree diseases in Australia, to help you identify problems in your own garden.

Myrtle rust

Myrtle Rust

Myrtle rust. Image from the Department of Primary Industries and Regional Development

Myrtle rust was first detected in New South Wales in 2010, after having slipped past the country’s biosecurity from an unknown South American source. The Australian Government created an emergency response plan that included the removal of diseased trees, fungicides, spore trapping, buffer zones, and quarantines7. Unfortunately the plan failed, and the disease quickly spread to North Queensland, Victoria, and Tasmania, where it causes problems for around 179 plant species from the Myrtaceae family, such as the eucalyptus, turpentine, paperbark, and bottlebrush. This disease is considered so severe that it may alter the composition of entire ecosystems, by attacking our most dominant plants.

Myrtle rust is caused by the fungal pathogen Puccinia psidii, which causes bright yellow powdery spots on a plant’s leaves, eventually turning brown or grey before killing the leaf. The spores created by the pathogen can be blown extremely long distances, contaminating plants, equipment, vehicles, and clothing.

There are a number of fungicides available for the control of myrtle rust, which a qualified arborist will use to treat the disease.

Pink disease

Pink Disease

Pink disease on a citrus tree, identified by white mycelium. Image from The Department of Primary Industries and Regional Development

Pink disease is caused by the prolific fungal pathogen Erythricium salmonicolor (also known as Phanerochaete salmonicolor), which can affect a huge range of trees. In Australia, it’s been found in trees such as the teak and African mahogany, as well as woody crops such as citrus, breadfruit, carambola, custard apple, durian, jackfruit, mango, mangosteen, and rambutan1.

Fungal pathogens spread by producing spores, which can be carried to the trees by wind, people, or animals. When Erythricium salmonicolor infects a tree, it grows within its bark and makes it swell, split, and eventually die—a process accelerated by rain and humidity. Its ruinous effects make it a serious problem for citrus farmers in Australia, who can lose swathes of grapefruit, lemons, lime, mandarin, and orange crops to the disease. The disease is a problem in New South Wales, Queensland, and the Northern Territory2.

The fungus produces different effects on the tree, depending on how much time it’s had to work. It can be broken down into three stages:

    1. Cobweb stage: a cobweb-like layer appears on the tree, which is mycelium produced by the fungus
    2. Nectar stage: orange structures start to develop, which produce spores
    3. Pink encrustation stage: cankers (dead sections) appear, and may be covered in pink fruiting structures

When removing a branch with pink disease, it’s recommended to burn them to prevent the disease from infecting other nearby plants.

Root rot

Root Rot

Root rot. Image from Washington State University

Root rot is the decay of a plant’s root system, and is difficult to diagnose and cure in established trees. A tree may have root rot if its leaves become wilted, small, or discoloured, if its branches are weak and decaying, if it develops cankers, or if its sap is oozing4. This disease is best diagnosed by a qualified arborist.

Root rot can be caused by poor drainage that leads to oversaturated soil, or infection from a fungal pathogen such as P.cinnamomi, Pythium, Fusarium, Rhizoctonia, Phytophthora, or Armillaria3. It is a serious disease that can kill a tree quickly, and for adult trees, can be incredibly difficult to cure.

Root rot can be discouraged by planting trees in well cultivated soil with proper drainage.

Peacock spot

Peacock Spot

Peacock spot disease. Image from Jeff Stephenson

Peacock spot is a disease that affects olive trees across the world. It’s caused by the fungal pathogen Spilocaea oleaginea, which can significantly damage farmer’s crops by blemeshing the fruit, delaying ripening, and reducing oil yield5. It can also damage the plant’s health for a long period after infection.

Spilocaea oleaginea typically thrives in mild to low temperatures, when moisture is able to accumulate on the plant’s leaves. The disease can be identified as sooty blotches on the plant’s leaves, which develop into black circular spots between 2.5 to 12mm in diameter, which can include a yellow halo6.

The disease is often treated by spraying the foliage with a copper compound (copper hydroxide, copper oxychloride, tribasic copper sulfate, or copper oxide) after its olives have been harvested, as well as in late winter for humid climates. 

Cypress canker

Cypress Canker

Cypress canker. Image from Global Urban Forest

Cypress canker is a disease that affects Australia’s exotic conifers, including the Monterey Cypress, Leyland Cypress, Castlewelland Gold, Naylor’s Blue, and more than 20 others. It can be caused by the fungal pathogens Seiridium cardinale, S. unicome and S. cupressi, which enter the plants through fissures in the bark, impede the tree’s sap system, and eventually kill its branches or entire trunk8. The fungal pathogens responsible for Cypress canker are spread by wind, rain, insects, and birds.

Cypress canker can be identified through dying branches that appear as “flags”, whose leaves will quickly yellow as they become starved of sap. Reddish cankers can also form where the fungus entered the tree, which exude a sticky resin.

Teratosphaeria (Kirramyces) leaf diseases 

Leaf Disease

Teratosphaeria leaf disease. Image from FABI

Teratosphaeria leaf diseases (one of many gum tree diseases) are a group of fungal pathogens that infect eucalyptus trees, causing their leaves to spot, blight, and die. The diseases are caused by four species of fungal pathogens: Teratosphaeria epicoccoides (formerly Kirramyces epicoccoides), T. viscidus (formerly K. viscidus), T. psuedoeucalypti, and T. corymbiae, each of which create unique symptoms when infecting a tree9.

Symptoms:

  • Teratosphaeria epicoccoides – small purple spots appear on the upper leaf, which expand and eventually cover its entire surface. Yellow lesions appear on leaf veins, and brown to black spores can appear on the underside of the leaf.
  • T. visciduscircular spots with red borders appear on the leaf, and black spore masses appear. Infection starts in the lower canopy, and if allowed to spread to the tree’s tip, can lead to 90% defoliation.
  • T. psuedoeucalyptibrown leaf spots appear with red margins, and black spore masses appear.
  • T. corymbiae – yellow or brown leaf spots appear.

References

  1. Pink disease, Business Queensland
  2. 2016, Pink disease of citrus: pest data sheet | Agriculture and Food, The Department of Primary Industries and Regional Development
  3. 2018, Root Rot in Trees: Frequently Asked Questions, PR Tree
  4. Root rot, Wikipedia
  5. Spilocaea oleaginea, Wikipedia
  6. Peacock Spot / Olive / Agriculture: Pest Management Guidelines / UC Statewide IPM Program (UC IPM), UC IPM
  7. Myrtle rust (Austropuccinia psidii) | Department of Agriculture, Water and the Environment, Department of Agriculture, Water and the Environment
  8. Cypress Canker, The Royal Botanical Garden Sydney
  9. Kirramyces leaf diseases, Business Queensland
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