Tree Disease In Australia | Signs & Solutions

Because they can live for centuries, people can often forget just how vulnerable trees can be, particularly when it comes to tree disease. Despite the Australian Government’s strict biosecurity, plenty of tree diseases have also made their way into the country, some of which are devastating to our precious flora. Subtle signs of tree disease can also be easy to miss. Here is a guide to identifying symptoms early and how to deal with those that are treatable.

General tree diseases

Anthracnose

Anthracnose is a fungal infection that can cause serious issues with various parts of a plant and can affect different plants in different ways. It thrives in damp areas, particularly under foliage.

In avocados, it can present as a fruit rot that remains latent until the fruit ripens. On the leaves of melons and cucumbers, small brown spots with halos appear which can later develop into reddish, round spots before the centres fall out. On watermelon leaves, the spots are irregular and angular and turn dark brown or black. Spots can also appear on the fruit and stems.

In mangos, young leaves are the most susceptible to infection, Small, black and irregular spots form before expanding to larger areas that eventually dry and fall out. Anthracnose can also cause blight of flowers and young shoots, and in mature fruit, cause dark brown to black spots on ripened fruit. 

In lettuces, symptoms appear as tiny black spots on the leaves before turning yellow and then brown.

Anthracnose can be managed with registered fungicides, but also via cultural methods. These include minimising periods of leaf wetness, pruning trees to aid airflow, humidity and by raking up infected material and disposing of this away from the subject plant and also inspecting regularly to identify disease symptoms early.

Cypress Canker

Cypress Canker is caused by a water-borne infection, but it can also be airborne, or spread from certain pruning materials that are used on other plants. What are “cankers?” They appear as a deep-red colour on branches known as flags, causing them to die rapidly, often as quickly as overnight. After that, the dead branches will turn yellow, releasing spores through the minuscule black fungus, which can then infect other branches.

If caught early enough, you may be able to salvage the tree if you remove the infected areas entirely. However, if it keeps spreading, you will have to remove the entire tree to prevent the disease from spreading to other trees. However, don’t replace the tree with another cypress as it will be even more prone to cankers and you will have to go through the same issue all over again.

Emerald Ash Borers

Emerald Ash Borers are small beetles that can cause significant damage if they aren’t dealt with swiftly. A telltale sign they are present is if you see the leaves on your ash tree turn yellow well before autumn, followed by the branches dying. These beetles ‘bore’ into the bark of a tree and then infect the tree while building a web of tunnels inside. All ash trees are susceptible to these pests and can cause serious damage.

Whilst certain fungicides can help to prevent infection, they’re not entirely reliable. Therefore, the only true “cure” for emerald ash borers is having the infected tree removed altogether.

Myrtle Rust

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 then created an emergency response plan that included fungicides, quarantines and the removal of diseased trees. Unfortunately, the plan failed and the disease quickly spread to other states, where it caused problems with several plant species from the Myrtaceae family, including the eucalyptus, bottlebrush and paperback.

Myrtle rust is caused by the fungal pathogen Puccinia psidii, which causes bright yellow powdery spots on a plant’s leaves. These eventually turn grey or brown before killing the leaf. The spores created by the pathogen can be blown extremely long distances, contaminating plants, clothing, vehicles and equipment. There are several fungicides available for the control of myrtle rust, which a qualified arborist will use to treat the disease.

Oak Wilt

Oak Wilt is a soil-borne fungus that is typically spread via beetles. Like many, this fungus is incredibly fast-acting and can take as little as a season to destroy an entire tree. It is incredibly infectious as well, spreading from oak tree to oak tree via the root system. Oak wilt can be spotted by the wilting, rust-coloured leaves that occur during the season in which they should all be green and luscious.

The moment symptoms are spotted, action should be taken immediately before the infection can spread. Managing this type of disease involves separating the tree by digging a trench around it, and cutting its roots off from the rest of the trees surrounding it.

Root Rot

Root rot is the decay of a plant’s root system which is difficult to diagnose and cure in established trees. Signs include leaves that become small, wilted or discoloured, branches that are weak and decaying, and/or if the plant develops cankers or its sap is oozing.

It can also be caused by poor drainage that leads to oversaturated soil, or infection from a fungal pathogen. Root rot is a serious disease that can kill a tree quickly, and for adult trees, can be incredibly difficult to cure. It can be discouraged by planting trees in well-cultivated soil with proper drainage. This disease is best diagnosed by a qualified arborist.

Specific tree diseases

Apple tree diseases

• Alternaria disease. The most common early sign of alternaria disease is purple lesions that later turn brown. Other signs include fruit drop, premature defoliation and brown spotting on fruit. The disease is best controlled by removing dead plant tissue, removing dropped leaves and fruit, sterilising tools between trees when pruning and applying a copper-based fungicide registered for use for apples.
• Apple scab. Young leaves show infection as velvety green spores that turn black as the fruit ripens. Treatment involves removing all leaves from around the tree after leaf fall as well as those mentioned for alternaria disease.
• Black spot. Black spot typically shows as black spots on the fruit’s surface that increase as the fruit grows. In terms of treatment, it is vital to plant trees where there is good air movement to minimise humidity. See the treatment for alternaria disease.

Citrus tree diseases

• Citrus gall wasp. Bulging and swelling on your citrus tree can indicate that it is afflicted with citrus gall wasp. This infection is caused by a reaction between the tree and the gall wasp’s spawn. The gall wasp will lay their eggs in the tree’s branches, and then the infection occurs. Once the eggs hatch, more eggs are typically laid, which causes the cycle to continue. The best method of control is to remove the swollen, bulging branches as soon as you spot them.
• Pink disease. This 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 has been found in trees such as the teak and African mahogany, as well as woody crops like custard apple, mango, jackfruit and rambutan. In particular, it can be a serious problem for citrus trees like lemon, orange, grapefruit, mandarin and different types of lime trees. Fungal pathogens spread by producing spores, which can be carried to the trees by wind, animals or people. When pink disease infects a tree, it grows within its bark and makes it swell, split and then eventually die. This process is accelerated by rain and humidity. The fungus has different effects on trees, depending on how much time it has 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 is recommended it be burnt to prevent the disease from infecting other nearby plants.

Fig tree diseases

Though they are considered very hardy trees, figs can also be affected by several other pests and diseases, including:

• Queensland fruit fly (Dacus tryoni). This is a major pest in many areas of NSW. Flies are small, brown/black flies with distinctive cream-to-yellow markings on their mid-sections. Females lay eggs in ripening fruit which then spoils. One solution is to try pheromone traps, which can attract and kill male flies. Fallen fruit should be destroyed.
• Fig blister mite (Aceria ficus). Colourless to white, blister mites attack the inside of fruit leaving rust-coloured dry patches that affect the fruit’s eating quality. They won’t be visible until fruits are harvested. If you find damaged fruit, destroy it to prevent subsequent fruits from being infected as they ripen.
• Fig rust and anthracnose. Both of these fungal diseases affect mainly coastal areas. Fig rust produces powdery yellow spots on the leaves. Anthracnose forms small brown to black spots, which develop into a larger patch of infection. With both diseases, leaves will turn yellow and then fall. As with most fungal diseases, copper-based fungicides are normally used for control.
• Fig mosaic virus. This affects leaf pigment and causes a mottled pattern on the leaf. Affected plants need to be destroyed.

Gum tree disease

Teratosphaeria leaf diseases are one of the more common eucalyptus tree pests and diseases, and they are a group of fungal pathogens that infect trees causing their leaves to spot, blight and then die. The diseases are caused by four species of fungal pathogens, each of which exhibits different symptoms.

• Teratosphaeria epicoccoides. Small purple spots appear on the upper leaf, which expands and eventually covers its entire surface. Yellow lesions appear on leaf veins, and brown to black spores can appear on the underside of the leaf.
• T. viscidus. Circular 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 per cent defoliation.
• T. psuedoeucalypti. Brown leaf spots appear with red margins, and black spore masses appear.
• T. Corymbiae. Yellow or brown leaf spots appear.

Olive tree disease

As one of the world’s oldest cultivated trees, olives are tough, versatile, drought-hardy and easy to grow (check out our guide on how to grow an olive tree, and you’ll discover just how easy it is). But despite their toughness, they can be affected by a disease called olive leaf spot. Also known as olive peacock spot and bird’s eye spot, it is a disease that affects olive trees across the world.

It is caused by the fungal pathogen Spilocaea oleaginea, which can significantly damage crops by delaying ripening, blemishing fruit and reducing oil yield. 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 a plant’s leaves, which develop into black circular spots between 2.5 to 12mm in diameter and can include a yellow halo.

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

Palm tree disease

Australia is home to over sixty known species of palms that flourish in its tropical and sub-tropical areas. However, despite the natural climate and environments that suit their healthy growth, they aren’t immune from a number of diseases. These include:

• Bud rot. This disease is caused by a type of fungus that attacks the heart fronds of palms causing them to shrivel up and die. This is particularly prevalent during summer. To treat this disease, chemicals can be applied given that the disease is still in its early stage. Gardening tools should also be sterilised to avoid fungi spreading.
• Fusarium wilt. This is another type of palm disease that is caused by fungus. Once it attacks the palm, it causes fronds to lose their green lustre, wilting and killing them in the process. This is a deadly disease and causes permanent damage to the plant. Once infected, there is no known cure, so the infected plant must be removed to prevent fungi spores from spreading. Fusarium is spread by soil particle movements.
• Ganoderma root rot. Originating in the US, this palm disease has spread to other parts of the world. To date, there is no known species of palm that is resistant to Ganoderma root rot. This disease is caused by the fungus Ganoderma zonatum entering a palm through a wound in the butt area. From there it rapidly makes its way into the tree, eventually rotting the wood.

Once the fungus has made its way into the centre of the plant, it forms a spongy, white mushroom-like growth that is referred to as a “conk”. This will form a horizontal disk on the bark of the palm. Once the “conk” has formed, it must be removed immediately from the area to avoid infecting other plants. All tools must also be sterilised after working with a diseased plant. The best way to avoid it is to prevent it from occurring in plants. Take care not to cause any cuts or wounds on the base of your palm, particularly when mowing.

Tree disease can occur in any plant, regardless of how well you take care of them. If you are unsure as to whether or not your trees are as healthy as they can be, consult an expert from your local nursery.

References

• 2024, Common Tree Diseases & Pests, Canopy Tree
• 2024, Common Fruit Tree Pests And Diseases, PlantNet
• 2024, Figs, Sustainable Gardening Australia
• 2024, Common Palm Diseases and How to Treat Them, Palms Online
• 2024, Anthracnose, Crop Science Australia