Myrtle Rust Identification Guide

What You Are Actually Looking For

Myrtle rust does not make a mushroom. Austropuccinia psidii is a rust fungus, and everything you will ever see of it in the field is a spore mass erupting through the skin of a living plant. So the search image is not a cap on a stalk in the litter; it is a colour on a leaf. Specifically, it is a shocking, almost artificial egg-yellow to orange-yellow powder, sitting in small round pustules on the softest, newest growth of a plant in the myrtle family.

Once you have seen it, the colour is hard to confuse. It is brighter and more saturated than pollen, more yellow than most other rusts, and it comes off on your fingertip as a fine dust.

Start With the Plant, Not the Fungus

The single most useful step in identifying this fungus is botanical. Austropuccinia psidii is restricted to the Myrtaceae, so before you look closely at a pustule, work out what you are standing in front of.

Myrtaceous plants share a set of features that are quick to check: leaves usually opposite or nearly so, margins entire rather than toothed, and, decisively, translucent oil dots visible when a leaf is held up against the light, usually with an aromatic scent when crushed. Eucalyptus, Corymbia, Melaleuca, Leptospermum, Kunzea, Syzygium, Backhousia, Rhodamnia, Rhodomyrtus, Metrosideros, Psidium and Callistemon all qualify.

If the plant is not a myrtle, the yellow rust you are looking at is a different fungus, no matter how similar it appears. That check alone eliminates the majority of misidentifications.

First Impression on a Shoot

Walk the outer canopy and the soft flush growth, not the old hardened leaves. Infection targets tissue that is still expanding: unfurling leaves, shoot tips, young stems, sepals, flower buds, and immature fruit.

The earliest visible stage is subtle. Look for tiny pale or chlorotic flecks a millimetre or two across, sometimes with a faint purplish or reddish halo, on the upper or lower leaf surface. Held to the light these flecks look like small windows in the leaf.

Within a few days those flecks lift and rupture, and bright yellow spore masses push out through the torn epidermis. This is the classic stage. Pustules are round to slightly irregular, one to three millimetres across, often crowded into clusters or arranged along a vein or a leaf margin. On stems and petioles they form raised yellow bumps that can girdle a shoot.

Reading a Lesion Step by Step

Take one infected leaf and work through it with a hand lens.

First, check which surface bears the pustules. Both surfaces can be affected, and pustules commonly appear on the underside with a matching discoloured patch above.

Second, look at the pustule rim. A rust pustule is a crater: the plant epidermis is torn back in a ragged ring around a mound of dry powder. Anything that sits on top of an intact leaf surface is not this fungus.

Third, look at the surrounding tissue. Fresh lesions have a chlorotic yellow halo; older lesions develop a purple-red to brown margin and eventually a grey-brown necrotic centre.

Fourth, look at deformation. Because young expanding tissue is attacked, infected leaves buckle, twist, cup and become distorted as they grow around dead patches. Shoot tips die back and the plant responds by pushing new side shoots, which are themselves attacked, giving a bunched, stunted, repeatedly killed-back appearance over a season.

Fifth, note the age sequence of colour. A colony runs bright yellow to dull khaki-brown as spores are shed, and later a darker, crustier brown spore stage may develop in the same lesions. A single branch can carry all stages at once, which is a useful confirmation.

The Spore Test, Instead of a Spore Print

There is no gilled cap here, so the equivalent procedure is simple. Hold a piece of dark card or black paper under an infected leaf and tap the leaf sharply. A myrtle rust pustule releases a visible puff of fine yellow dust that settles as a clear yellow film on the card. Draw a fingertip across a pustule and it stains yellow.

Under a microscope the urediniospores are pale yellow, broadly ellipsoid to obovoid, roughly 14 to 20 by 12 to 16 micrometres, and covered in fine spines, which makes them look stippled at high magnification. The later dark stage produces two-celled teliospores. Note that a pustule that no longer releases powder has already discharged, and the lesion will look brown and spent.

Substrate, Season and Conditions

The fungus is native to South and Central America and now occurs across Australia, New Zealand, New Caledonia, Hawaii, parts of Asia, Africa and the Pacific.

Weather drives everything. Infection needs several hours of continuous leaf wetness, high humidity and mild temperatures, broadly in the mid teens to mid twenties Celsius, together with a flush of soft new growth. In practice that means warm humid nights after rain in spring and autumn in subtropical regions, and near-continuous cycles in the wet tropics. Sheltered, shaded, poorly ventilated plantings and regrowth thickets show it before exposed windy sites do.

Record host species, plant part, date and location when you find it. Distribution is still expanding and host-by-host records are genuinely useful to local plant-health recorders.

Look-Alikes and How to Separate Them

Other rust fungi. Many rusts produce yellow to orange pustules. Separate them on the plant: rusts are highly host-specific, and only Austropuccinia psidii attacks Myrtaceae in most regions. The gall-forming Uromycladium rusts, for instance, occur on wattles and produce brown, distorted galls rather than yellow powder on soft leaves.

Powdery mildew. White to pale grey, superficial, felty, and it wipes cleanly off an intact leaf surface. Rust erupts from beneath and leaves a ruptured rim.

Sooty mould and scale insect residue. Dark grey to black films growing on sticky honeydew, superficial and peelable, often with visible insects on stems.

Pollen deposits. On calm days myrtaceous flowers shed yellow pollen onto leaves below. Pollen is loose, evenly scattered, unrelated to lesions, and rinses off without leaving a scar.

Leaf spot and anthracnose fungi. These produce dark brown to grey spots, often with concentric zoning, and any fruiting structures are minute black dots embedded in the dead tissue, not yellow powder.

Nutrient deficiency, sunburn and spray scorch. These give diffuse, symmetrical yellowing or bleaching, often between veins or along margins, with no pustules, no powder and no torn epidermis under a lens.

The clinching combination is a myrtaceous plant, soft young tissue, raised pustules that have burst through the epidermis, and bright yellow powder that transfers to card and skin.