Cedar-Apple Rust
Gymnosporangium juniperi-virginianae
Also called cedar apple rust gall, juniper-apple rust, gelatinous rust gall
A rust fungus that alternates between junipers, where it forms brown galls that push out orange gelatinous horns in spring rain, and apples, where it makes bright orange leaf spots.
Overview
Cedar-Apple Rust (Gymnosporangium juniperi-virginianae) is not a mushroom at all but a rust fungus, a member of the Pucciniales, and it is one of the most visually dramatic. Its life cycle requires two unrelated hosts: a juniper, usually eastern red cedar (Juniperus virginiana), and a member of the apple group, usually apple or crabapple (Malus).
On the juniper it produces hard, brown, roughly spherical galls, from golf-ball size downward, pocked with circular depressions. After warm spring rains each depression extrudes a bright orange, gelatinous, tongue-like horn, and a heavily galled tree can look as though it has burst into orange flowers overnight. The horns collapse to slime when they dry and can revive with the next rain.
On the apple side the fungus makes small, vivid yellow-orange spots on leaves and sometimes fruit, which later develop tiny tube- or cup-like structures on the underside. The two phases look so unalike that they were long described as separate organisms.
How to identify it
Juniper galls
Hard, woody-brown, globose to kidney-shaped galls 1-5 cm across, borne on twigs and small branches, with a surface covered in shallow circular pits. Immature galls are green and inconspicuous; they mature over roughly eighteen months.
Telial horns
After prolonged spring rain, each pit extrudes a gelatinous horn 1-2 cm long, bright orange to orange-brown, tapering and often curved, soft and slimy to the touch. These shrivel to thin brown threads in dry air and re-expand with the next soaking.
Apple-leaf phase
On Malus, small circular spots appear on the upper leaf surface, at first pale yellow, becoming vivid orange with a darker or reddish border and often a scatter of black dots at the centre. On the leaf underside, and on infected fruit and twigs, clusters of short pale tubes or fringed cups develop later in the season.
Spores
There is no spore print. The orange horns produce teliospores that germinate in place to release wind-borne basidiospores, while the apple phase produces spermatia and then chains of aeciospores in the tubular structures. Colour, not print, is the field character.
Where and when it grows
The fungus occurs wherever eastern red cedar and other susceptible junipers grow within a few kilometres of apples, crabapples and related rosaceous trees. It is common across eastern and central North America, from southern Canada to the Gulf and west into the prairie states, and follows both hosts into planted landscapes, orchards, hedgerows and old fields.
Juniper galls are visible all year but only become conspicuous in spring, typically April to June, when repeated rain triggers the orange horns. The apple-leaf spots develop through late spring and summer, becoming most obvious by mid to late summer before leaf fall.
Ecological role
Gymnosporangium juniperi-virginianae is an obligate parasite of living plant tissue and cannot be grown apart from a host. It is heteroecious, meaning it must alternate between two host genera to complete its cycle, and macrocyclic, passing through several distinct spore stages.
Basidiospores released from the gelatinous horns on juniper are carried by wind to expanding apple leaves, where they infect and produce the orange leaf spots and, later, aeciospores. Those aeciospores blow back to junipers in late summer, infecting needles and twigs and initiating new galls that take about a year and a half to mature. The fungus persists year to year in the perennial juniper tissue.
Similar species
Related rusts on juniper are separated by the shape of the fruiting structure and by the second host. Gymnosporangium globosum, the hawthorn rust, makes smaller, more irregular galls with shorter, blunter orange horns and alternates with hawthorn. Gymnosporangium clavipes, the quince rust, does not form discrete round galls at all but produces elongated swellings and spindle-shaped cankers along juniper twigs, with the alternate stage causing distorted fruit on quince, hawthorn and apple.
The orange gelatinous horns can suggest a jelly fungus such as Tremella mesenterica or Dacrymyces, but those grow on dead wood or bark as free-living lobed masses, never emerging from pits in a hard woody gall on a living twig. On the apple side, the bright orange bordered spots with black centres separate this rust from apple scab, which makes olive-brown velvety blotches, and from frogeye leaf spot, which forms brown spots with concentric zoning.