Rose Rust
Phragmidium mucronatum
Also called phragmidium rust, orange rose rust
A rose-specific rust that speckles leaf undersides with orange pustules from late spring, replacing them with sooty black telia by autumn.
Overview
Rose rust belongs to the genus Phragmidium, a group of rusts in the Phragmidiaceae confined to the rose family and recognised microscopically by long, many-celled teliospores borne on a stalk. Phragmidium mucronatum is one of the commonest species on cultivated and wild roses, alongside P. tuberculatum, P. rosae-pimpinellifoliae and others; separating them reliably needs a microscope, and in the field they are usually treated together as rose rust.
The fungus is autoecious, completing every stage on rose without any alternate host. In spring the earliest sign is bright orange, slightly swollen spots on petioles, stems and the undersides of young leaves. Through summer these are followed by powdery orange uredinia on the leaf undersides, matched by yellow flecks above.
By late summer and autumn the pustules darken dramatically to black as telia replace them, so the same leaf can carry orange and black pustules side by side, a combination that makes the fungus easy to name at a glance.
How to identify it
Spring stage
Orange to bright yellow-orange cushions appear on petioles, young stems, leaf veins and sepals, sometimes slightly swelling or distorting the tissue. These are the aecia and spermogonia, and they can be conspicuous before any leaf spotting shows.
Summer pustules
The underside of the leaflet carries scattered to dense, round, powdery pustules under 1 mm across, orange to rusty orange, that dust off onto a finger. The corresponding upper surface shows small pale yellow flecks that darken and merge as the leaf ages, and heavily colonised leaflets yellow and fall.
Autumn telia
From late summer the pustules turn from orange through dark brown to sooty black, becoming firmer and less powdery. Black telia are typically most abundant on the underside of the oldest leaves and along petioles, and they persist on fallen leaves and stems.
Spores
No spore print applies. Urediniospores are orange, ovoid and finely spiny. Teliospores are the distinguishing feature of the genus: cylindrical, dark brown, divided by cross-walls into several cells (often five to nine), with a thickened apex and a long hyaline stalk.
Where and when it grows
The fungus grows on wild roses in hedgerows, scrub, woodland edges and dunes, and on garden and cut-flower roses in beds, borders and nurseries. Hosts include dog rose, field rose, burnet rose and many cultivated hybrids, with species varying widely in how readily they are colonised.
It is widespread across Europe and temperate Asia and has been carried with cultivated roses to North America, Australasia, southern Africa and South America. Activity begins in late spring, builds through humid summers with cool nights and heavy dew, and continues into autumn until leaf fall. Sheltered, crowded, poorly ventilated plantings and long periods of leaf wetness favour it strongly.
Ecological role
Phragmidium mucronatum is an obligate biotroph restricted to Rosa and completes all five spore stages on that single genus, so no alternate host is involved. It overwinters as teliospores on fallen leaves and on infected stems and thorn bases; these germinate in spring to produce basidiospores that colonise the emerging shoots and start the aecial stage.
Through summer the uredinial stage repeats every one to two weeks under humid conditions, allowing rapid buildup within a planting. Progressive defoliation reduces the shrub's growth and flowering, but the fungus does not kill its host, instead persisting in the same rose population year after year. Wild rose thickets act as long-term reservoirs from which garden plantings are recolonised.
Similar species
Rose black spot (Diplocarpon rosae) produces flat, black, feathery-edged spots on the upper leaf surface with yellowing around them and never leaves orange powder beneath. Powdery mildew (Podosphaera pannosa) forms a white superficial felt on young leaves and buds. Downy mildew on rose causes angular purplish blotches bounded by veins with a faint greyish growth underneath, again with no orange pustules.
Among true rusts, Phragmidium tuberculatum is nearly identical in the field and separated by teliospore apex shape and wall ornamentation under a microscope, while rusts on nearby brambles and raspberries (Phragmidium violaceum, Phragmidium rubi-idaei) look similar but occur only on Rubus, so host identity settles it.