Cedar-Apple Rust Identification Guide

Two Fungi in One, and Why That Matters

This rust looks like two unrelated organisms, and the identification hinges on knowing which one you are standing in front of. On juniper it is a hard brown ball on a twig that erupts into orange jelly after spring rain. On apple and crabapple it is a scatter of bright yellow-orange spots on leaves that later grow fringed tubes underneath. There is no cap, no gills and no stipe anywhere in the life cycle, so the identification is built from gall shape, horn shape, spot pattern and, critically, host identity.

Because the fungus must alternate between the two hosts, the presence of both an eastern red cedar and a Malus within a few kilometres is part of the evidence, not just background information.

Reading the Juniper Gall

Walk a hedgerow or field edge with Juniperus virginiana in it and look along the smaller branches. The galls sit on twigs, usually within a few centimetres of the tip, and are 5-50 mm across, most commonly the size of a hazelnut to a walnut.

A mature gall is hard, woody in feel, and globose to kidney-shaped, with a brown to chocolate or reddish-brown surface. The single best character is the arrangement of the surface: it is covered in evenly spaced circular depressions two to four millimetres across, each with a small central papilla, so that the whole gall looks like a golf ball with dimples that have been drilled slightly too deep. Those depressions are the sites from which the horns will emerge.

Galls have a defined age structure worth reading. A gall formed during the summer after infection stays small, green-brown and inconspicuous through its first year, hardens over winter, and only reaches full size and dimpling in its second spring. After it has produced its horns it dies and stays attached, greying, for a year or more. Finding tiny fresh galls, dimpled mature galls and grey spent galls on the same tree is normal and confirms an established population.

The Telial Horns

This is the stage everyone photographs. After a warm spring rain, sometimes within hours, each depression extrudes a gelatinous horn: a tongue-shaped or wedge-shaped orange to orange-brown tendril, 10-20 mm long and a few millimetres wide, which swells as it absorbs water until the gall carries a full crown of them and looks like a small orange sea creature stranded on a branch.

Record three things. Length, because the horns of this species are long relative to the gall. Shape, because they are strap-like and taper towards the tip rather than being short cushions. And behaviour: as the weather dries, the horns collapse into shrivelled threads plastered to the gall, and they will rehydrate and swell again with the next rain, repeating for several weeks.

Under a lens, take a scrape of the jelly and mount it in water. The teliospores are two-celled, ellipsoid to spindle-shaped, yellow-brown, with a germ pore in each cell and a long gelatinous stalk that is what swells so dramatically. In fresh wet material some will already have germinated into short four-celled basidia bearing tiny colourless basidiospores.

Reading the Apple Stage

From late spring the Malus host begins to show infection. On the upper surface of the leaves, circular spots 3-15 mm across appear, at first pale yellow and then deepening to bright yellow-orange or with an orange centre and a yellow halo, often with a reddish rim. Hold a spot up to the light and it is translucent.

Examine the centre of a spot with a lens. Minute black dots are scattered there, and in humid weather each exudes a tiny sticky droplet. These are the spermogonia, the sexual stage, and they are visible weeks before anything appears underneath.

Several weeks later, turn the leaf over. Beneath each spot is a cluster of tubular structures 3-6 mm long, pale buff to cinnamon, standing perpendicular to the leaf. Each tube splits lengthwise into narrow strips that curl outward, giving a fringed, shredded, cage-like appearance, and orange-brown powder is released from within. These are the aecia, and their form is the best single character on this host. A scrape of the powder under a microscope shows globose to broadly ellipsoid aeciospores with thick, densely warted walls and orange contents.

Spots can also form on the petioles, on green shoots and, less often, on the fruit surface, where they show as slightly raised orange-flecked patches.

Why There Is No Spore Print

Rusts have no hymenium that sheds spores downwards, so a print in the agaric sense does not exist. Substitute two direct preparations. Scrape a small amount of gelatinous horn material into a drop of water on a slide and look for stalked two-celled teliospores. Then, on the apple host, tap or scrape aecial powder onto a second slide and look for the thick-walled warted aeciospores. The contrast between those two spore types on two different hosts is the most convincing confirmation available without sequencing.

Season and the Two-Year Cycle

The timing is part of the identification. Galls swell and produce horns from roughly April to June, triggered by warm rain, and the horn display is short-lived and weather-dependent. Basidiospores released from the horns are fragile and travel a few hundred metres to a few kilometres, infecting expanding apple leaves in late spring. Spots become obvious on apple through May to July, spermogonia mature within them, and the aecia ripen from July into September. Aeciospores blow back to juniper from late summer into autumn, infecting new twigs, and the resulting galls take until the following spring to mature.

The consequence is a useful diagnostic fact: this fungus cannot spread from juniper to juniper, nor from apple to apple. If you see a juniper gall, an apple host is somewhere nearby, and vice versa.

Separating the Look-alikes on Morphology

Gymnosporangium globosum, cedar-hawthorn rust, is the closest match on juniper. Its galls are smaller, rarely over 20 mm, more irregular and often lumpy rather than neatly spherical, and their horns are markedly shorter, blunter and wedge-shaped, often a duller orange-brown. Its galls may also survive to produce horns in more than one season, which those of G. juniperi-virginianae do not. Its alternate hosts are mainly hawthorn and serviceberry, with apple less often, and the leaf spots are smaller with shorter, less conspicuously fringed aecia.

Gymnosporangium clavipes, cedar-quince rust, does not make a discrete round gall at all. Instead it forms perennial spindle-shaped swellings along juniper twigs and small branches, with the bark cracking and short orange cushions or ridges of telia erupting directly along the swelling. On the pome host it attacks fruits, twigs and thorns rather than producing crisp leaf spots, and its aecia are long, white and conspicuously fringed.

Gymnosporangium nidus-avis produces witches' brooms, dense proliferations of shortened juniper shoots, with telia among the foliage rather than on a discrete gall.

On juniper the other confusion is not fungal at all. Insect galls, including those made by juniper tip midges, are soft, made of clustered distorted leaves, and lack both the regularly spaced circular depressions and the capacity to produce jelly horns after rain.

On apple leaves, keep this rust apart from apple scab, which forms olive-brown to blackish velvety blotches with diffuse edges and no orange colour, and from frogeye leaf spot, which gives brown lesions with purple margins and small dark specks but no tubular structures underneath.

Working Checklist

On juniper: hard brown twig gall with evenly spaced circular depressions, producing long strap-shaped gelatinous orange horns after warm spring rain, then dying. On apple: bright yellow-orange upper leaf spots with black spermogonia at the centre, later bearing clusters of 3-6 mm fringed tubular aecia on the underside. Two-celled stalked teliospores in the jelly, warted orange aeciospores from the tubes. Both hosts within a few kilometres.