Myrtle Rust
Austropuccinia psidii
Also called guava rust, eucalyptus rust, ohia rust
A rust of the myrtle family that coats expanding shoots, buds and young leaves in startling egg-yolk yellow spore powder, later leaving grey-brown scarred and distorted tissue.
Overview
Myrtle rust is caused by Austropuccinia psidii, formerly placed in Puccinia and also encountered under the names Uredo rangelii and guava rust. It is the only rust known to attack the myrtle family (Myrtaceae) on a broad scale, and its host list is exceptionally wide, running to hundreds of species across genera such as Eucalyptus, Corymbia, Melaleuca, Syzygium, Psidium, Metrosideros, Leptospermum, Rhodomyrtus and Backhousia.
The fungus is unmistakable when sporulating. Rather than discrete pustules on old leaves, it colonises soft, actively expanding tissue and produces masses of brilliant egg-yolk to lemon-yellow urediniospores that can cover an entire young leaf, bud, flower or shoot tip.
Native to South and Central America, it has spread over recent decades to Florida and the Caribbean, Hawaii, Japan, China, South Africa, Australia, New Zealand and parts of Southeast Asia, becoming one of the most significant biosecurity concerns for native Myrtaceae-dominated vegetation.
How to identify it
Early signs
The first signs on expanding leaves are tiny chlorotic flecks or purplish-brown spots, often on the leaf edge or tip where growth is softest. Within days these produce erupting pustules.
Sporulating stage
Pustules are bright yellow to orange-yellow and powdery, appearing on either leaf surface, on petioles, on stems, on flower buds, on sepals and on young fruit. Individual pustules are under 1 mm but they coalesce rapidly into large mats, and a shoot tip can become entirely coated. The colour is notably more yellow than the rusty orange of most cereal and rose rusts.
Later stage
As pustules age they turn dull, greyish or brown, and the underlying tissue dies. Repeatedly infected shoots develop puckered, twisted, distorted leaves, shoot tip death, bushy proliferation of side shoots, and defoliation. Old lesions on stems can persist as roughened grey scars.
Spores
No spore print applies. Urediniospores are pale yellow, ovoid to pear-shaped and finely echinulate; teliospores are two-celled and produced sparsely, often in the same lesion late in its life. Sporulation on young tissue only, in that particular yellow, is the practical field key.
Where and when it grows
The fungus is confined to hosts in the Myrtaceae, so it is found in eucalypt forests and woodlands, coastal scrub and heath, tropical rainforest margins, Metrosideros forests on Pacific islands, guava orchards, and gardens and nurseries wherever ornamental myrtles are grown.
Infection requires soft new growth plus several hours of leaf wetness at mild to warm temperatures, so activity peaks after rain in spring and autumn in subtropical climates and can continue year-round in the humid tropics. Because it depends on flushing growth, the rust is most conspicuous on regenerating vegetation, coppice regrowth, seedlings and nursery stock.
Ecological role
Austropuccinia psidii is an obligate biotroph and, unusually for a rust, appears to complete its cycle on a single host family without an alternate host, spreading through repeated uredinial generations. Its spores are small, dry and readily carried by wind, on animals, on clothing and on plant material, which explains its rapid intercontinental movement.
Ecologically its impact runs through the whole plant community. By repeatedly killing new shoots, flowers and seedlings it can suppress regeneration in susceptible species, shifting the composition of Myrtaceae-dominated vegetation and affecting the birds, insects and mammals that depend on those trees and shrubs for nectar and structure. Susceptibility varies enormously between and within species, so the fungus also exerts strong selection on host populations.
Similar species
Few things imitate the vivid yellow sporulation. Puccinia psidii records from older literature refer to the same fungus under its previous name, as does Uredo rangelii. Other rusts occurring on nearby vegetation, such as Puccinia species on grasses or Phragmidium on roses, are duller orange to rusty brown and occur on entirely different host families.
Non-rust confusions include eucalypt leaf spot fungi such as Teratosphaeria and Quambalaria, which cause purplish or grey blotches and shoot dieback but produce no yellow powder, and insect damage or sooty mould, which leave dark or sticky residues. Any bright yellow powdery growth on soft new Myrtaceae growth is almost certainly this species.