Identifying Diseases: Target Leaf Spot (Aulographina eucalypti)

Targeted Trees:

The most common and distinctive disease that attacks most eucalypt species in south-eastern Australia and is most often found (not exclusively) in the Brisbane and Cairns area. Target leaf spot (Aulographina eucalypti) often targets the following trees:

• Shining gum (E. nitens)
• Sydney blue gum (E. saligna)
• Dunn’s white gum (E. dunni)
• Rose gum (E. grandis)
• Smooth-barked apple (Angophora costata)
• Blackbutt (E. pilularis)
• Gympie messmate (Eucalyptus cloeziana)

It can also on occasion affect a variety of other non-eucalyptus native trees, although this is uncommon.

Symptoms:

The most distinctive of all the leaf spot diseases that target eucalypts across Queensland and New South Wales. Target spot usually starts as singular spots ranging in size between 1-15mm in width and takes on a circular shape. The fungi have a yellowish-brown, sometimes dark brown colour with a concentric pattern in which it gets its name.

They tend to form around the center of the leaf first and will tend to congregate into a mass if the host leaf is heavily infected. These spots are often raised above the surface of the leaf and have a corky texture but very rarely penetrate to the other side of the leaf.

Younger, less developed leaves are more at risk to infection than the older, hardened leaves. However, the lesions tend to be larger and more easily visible on the older infected leaves that are found lower in the canopy, especially in the areas well shaded from the sun. Leaves that find themselves heavily infected are often shed prematurely which subsequently leads to a noticeable reduction in the fullness of the lower crown as a result.

Causes:

Outbreaks of Target leaf spot (Aulographina eucalypti) occur most frequently during periods of high humidity or rainfall while within its optimal temperature window. Since sporulation (the release of spores) only occurs while the fruiting bodies are both wet and within its ideal temperature range of 15 to 20 degrees Celsius, this leaves spring and early summer as the most optimal time for the fungal infection to spread.

Lesions caused from Target leaf spot on lightly infected leaves, stems and fallen leaves are all potential avenues of further spreading the disease to fresh trees, which is why it is absolutely vital to ensure any green waste is disposed of correctly as the lesions caused by the fungus can survive for an incredibly long amount of time.

Management & Treatment:

Most attacks by Target leaf spot (Aulographina eucalypti) are usually sporadic and eventually pass without any intervention needed, especially with established trees. However, if the disease returns frequently every single year, measures should be taken to identify the cause and take the appropriate measures.

Since the disease usually passes between leaves in close proximity to one another, reducing the crowding of the canopy especially with trees that have overlapping canopies can dramatically reduce the risk of infection. Pruning is an effective option to handle this course of action but should be left to trained professionals, especially in the case of tall, well established trees that require climbing to reach the canopy.