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6 Most Common Tree Diseases In Australia

gall wasp on tree branch

Trees, like all living things, are unfortunately not immune to diseases. Whether it’s a disease that has travelled from overseas or a pest infestation, a diseased tree can quickly die, spread infection or become a hazard if left untreated. At Jim’s Trees, we’re trained arborists and aren’t strangers when it comes to treating diseased trees. Here are our tips on identifying disease in your backyard trees and how to treat them.

How to spot a sick tree

Good tree maintenance involves more than just watering and mulching. Taking the time to regularly inspect and prune your trees can make a huge difference in their health and longevity. We recommend inspecting your tree every few weeks so any changes are immediately noticeable. 

The common signs of a diseased tree include:

  • The presence of pests, fungi and insects on your tree bark, trunk, leaves and branches
  • Cracking, peeling and any other changes in the bark of your tree
  • Discoloured or changes in foliage appearance 

Disease isn’t the only cause when it comes to sick trees. Sometimes a tree becomes unhealthy due to changes to its environment, soil, or watering and nutrients. If you are unsure why your tree is sick, it’s best to ask the experts by arranging an arborist to assess your tree. This will prevent you wasting money and time treating your tree for a disease it doesn’t have.

Most common tree diseases

Even with our strict biosecurity measures and regular maintenance, tree diseases can still occur in Australian trees. Most diseases tend to stem from infections spread by fungi and pests. As arborists, we regularly interact with sick trees and come across a variety of diseases. The following six tree diseases tend to be the most common in Australia.

1. Root Rot

Root Rot infects the roots of a tree, weakening its root system and stumping growth. As it is a decaying disease, if left untreated Root Rot will kill your tree. Poorly drained or waterlogged soils is a key cause of Root Rot as it prevents your tree from properly absorbing sunlight and other necessary nutrients. Root Rot can also occur with the presence of fungi which thrives in overwatered soil and attacks weakened soil. 

It may be initially tough to spot Root Rot as it begins underground with the root system, however there are some symptoms that your tree may display.

Common signs of Root Rot include:

  • Small, pale, wilting, yellow or brown leaves
  • Thinning canopy 
  • Stunted or very slow growth 

While prevention is the best method, you can treat Root Rot by pruning infected roots and removing the tree entirely in order to prevent the spread. Some chemicals, such as Methyl bromide can be effective in reducing the spread of the disease, but will not kill it entirely.

2. Cypress Canker

Cypress Canker tends to affect introduced conifer trees and is a fungi that spreads through insects, water and wind. Cypress Cankers tend to thrive in moist, wet conditions, with waterlogged or nutrient trees being easy targets. A Cypress Canker will girdle branches, cutting off foliage and branches from the trunk. This will kill the tree branch by branch. 

Cypress Cankers are instantly recognisable by their sunken, lens shaped and reddish tint appearance. Other symptoms that reveal your tree has been infected by a Cypress Canker include:

  • Ringbarking 
  • Circular small black dots at the surrounding a canker
  • Branches yellowing and dying 
  • Branch death 

There is currently no officially known cure for the Cypress Canker, however basic maintenance and prevention can make a significant difference in the spread. 

We recommend:

  • Pruning infected branches, a minimum 10 centimetres below a canker
  • Removing and destroying infected trees 
  • Planting trees in non-infected areas with low nitrogen levels in the soil

3. Myrtle Rust

Myrtle Rust comes from the exotic fungus Austropuccinia psidii and infects trees of the Myrtaceace family including Australian native species like tea tree, bottlebrush and eucalyptus. Myrtle Rust prevents new growth, weakens the immune system of a tree exposing to increased risk of other diseases, and can kill a tree. 

You can spot Myrtle Rust by its:

  • Brown/grey spots with red/purple halos on leaves, fruits, and flowers 
  • Yellow spores 

Due to its highly infectious nature, it’s best to call in an arborist who can apply fungicide without spreading the disease which is easily transmittable through clothes, wind, equipment and water.

4. Citrus Gall Wasp

The Citrus Gall Wasp is a native insect which creates galls in tree branches to hatch its larvae. These galls weaken citrus trees, reducing fruit size and occasionally killing branches. You can easily spot the disease by the sudden emergence of a gall on your tree branch, the gall will grow in size if left untreated.

It’s best to prune your galls from April to June, and burn or bake the infected branch in the sun to avoid the wasps spreading to other trees. Regularly check for any light green shots growing and right angles, and inspect for any galls. You can also apply petroleum spray oils and other pesticides to deter egg laying.


5. Pink Disease

Pink Disease is known to infect a range of woody and citrus trees across Australia. Preferring wet tropical conditions, Pink Disease shows up on trees as various fungi types:

  • A layer on vegetable mycelium on branches
  • Orange or pink spores on cankers and tree branches

If your tree is infected by Pink Disease, it will also suffer from branches and bark splitting, stunted growth and may cause the entire tree to die. We recommend removing branches with the fungi and burning them to avoid the spread of infection. 

6. Peacock Spot

If you have an olive tree, you’re likely familiar with the menace that is Peacock Spot. This fungi will lead to your tree experiencing stunted growth and delayed fruit development. Common symptoms are sooty blotches on your leaves which then turn into black spots with a yellow halo.

It’s best to treat Peacock Spot with a copper spray, monitor soil levels, remove any infected branches and ensure trees have sufficient airflow. 

Ask the tree professionals

If you’re unsure whether your tree is suffering from one of the diseases listed above, then get in touch with the team at Jim’s Trees. We’ll be able to assess and diagnose your plant, recommend a treatment plan and perform any necessary maintenance or removal.


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