Juniper-Hawthorn Rust Identification Guide

Two Places, Two Faces

Gymnosporangium globosum cannot be identified the way a mushroom is identified, because it is not a single structure standing on the ground. It is a rust that spends its life shuttling between two unrelated woody plants, and it looks completely different on each. On juniper it appears as a hard brown gall that sprouts gelatinous orange horns for a few days each spring. On hawthorn it appears as bright orange leaf spots that grow a fringe of tiny tubes on their undersides in late summer. Confident identification usually means reading one stage well and knowing what the other host in the neighbourhood is.

The setting that produces both stages is ordinary and often suburban: a hedgerow, a park, a cemetery, a shelterbelt or a garden where an eastern red cedar or another juniper stands within a few hundred metres of hawthorns. Spores travel further than that, but the heaviest infection is always close-range, so the first move on finding signs on one host is to turn around and look for the other.

The Juniper Stage, Step By Step

Work on the juniper first if it is spring. Scan the small twigs, not the trunk, at eye level and below. The galls are firm, woody-cored swellings attached to the twig, mostly 5 to 15 mm across, irregularly rounded rather than perfectly spherical, and chocolate to reddish brown with a dry, hard, minutely dimpled surface. Each dimple is a shallow depression from which a horn will later emerge. Galls persist for more than one season and can be found at any time of year, so a dry brown lump in December is still a usable record.

After a warm spring rain the galls transform within hours. From those depressions push out telial horns: short, blunt, wedge- or tongue-shaped columns of orange to orange-brown gelatinous tissue, generally only a few millimetres long and rarely exceeding a centimetre even when fully swollen. Wet, they are slippery and jelly-like and can be drawn into a thread between finger and thumb. Dry, they collapse into stiff waxy scales that look like almost nothing, then re-swell with the next rain, often through two or three cycles.

Note the proportions carefully: on this species the gall is small and the horns are short and stubby, and the swollen mass never approaches the extravagant tentacled appearance of some relatives. Note also that the gall sits on a twig that continues past it, alive, rather than being a spindle-shaped swelling of the branch itself.

The Hawthorn Stage, Step By Step

From early summer, examine hawthorn leaves. Infection appears as rounded spots on the upper surface, 3 to 10 mm across, at first yellow, later deepening to orange with a distinctly redder rim. Hold a leaf to the light and the spot glows. Under a lens the upper face of each spot carries a scatter of minute, dark, pinprick-like bodies. These are spermogonia, the receptive structures where the rust exchanges nuclei, and they often exude a sticky droplet that dries into a shiny bead.

Now turn the leaf over, which is where the identification is actually made. On the lower face, directly beneath the spot, a cluster of aecia develops in mid to late summer. In this genus they take a very distinctive form: slender cylindrical or slightly tapering tubes, 2 to 5 mm long, pale cinnamon to buff, standing more or less erect and splitting lengthwise down their sides so that the walls curl back in narrow shredded strips. The classic description is a fringed tube or a miniature opened lantern. Inside, and dusting off freely, sits a mass of powdery orange-brown aeciospores.

On heavily infected trees the same structures appear on leaf petioles, green twigs and occasionally on the developing haws, always with the same tubular, split-sided form.

Reading The Spores

There is no spore print to take here, and no gill or pore surface to work from, so substitute a slide. Two preparations are worth making.

Scrape a fragment from a swollen orange horn on juniper into a drop of water. The teliospores are two-celled, brown to golden, rounded to elliptic, divided by a cross wall and mounted on a long colourless stalk that swells enormously in water — that swelling is what makes the horns gelatinous in the first place. Under a coverslip you may watch them germinate within an hour or two, throwing out short four-celled promycelia that bud off tiny colourless basidiospores. Those basidiospores are the ones that drift to hawthorn.

Scrape a little orange powder from the hawthorn tubes instead and you get aeciospores: roughly spherical to broadly elliptic, thick-walled, pale orange-brown, and covered in fine warts. They are noticeably larger and coarser than the basidiospores. Alongside them you will find the distinctive elongated cells of the aecial wall, which have thickened, patterned inner faces.

Substrate And Season

The juniper hosts are eastern red cedar and its relatives, including Rocky Mountain juniper and creeping juniper, with ornamental cultivars in gardens often heavily affected. The alternate hosts sit in the rose family and are dominated by hawthorns; apple, crabapple, pear, serviceberry and quince carry the genus generally, but hawthorn is the signature partner for this species.

The calendar is tight and worth memorising. Galls: visible year-round. Horns: extruding during warm rainy spells in mid to late spring, each episode lasting only a few days. Leaf spots on hawthorn: appearing from early summer. Tubular aecia beneath them: maturing mid to late summer, releasing spores that blow back to juniper in late summer and early autumn to start new galls, which then take the better part of two years to mature. Nothing happens on the juniper side in autumn, so an October search finds only old dry galls.

Separating It From Look-Alikes On Structure Alone

Cedar-apple rust, Gymnosporangium juniperi-virginianae, is the constant confusion. Its juniper galls are much larger, commonly 1.5 to 5 cm, distinctly globose and evenly dimpled like a golf ball, and their telial horns are long, slender, tapering and often 2 to 5 cm when swollen, hanging and curling in orange masses that make the whole shrub conspicuous from a distance. Its aecia develop on apple and crabapple. Small irregular gall plus short blunt horns points to the hawthorn rust; big round gall plus long tapering horns points to the apple rust.

Quince rust, Gymnosporangium clavipes, does not build a discrete detachable gall at all. Instead the juniper twig or branch itself swells into an elongated spindle, sometimes many centimetres long, and the telial tissue emerges as a low orange crust along cracks rather than as distinct horns. Its aecia concentrate on the fruits and green stems of the alternate host, where they form short, cup-like structures with ragged margins rather than long tubes, and infected haws become distorted and swollen.

Pear trellis rust, Gymnosporangium sabinae, produces spindle-shaped perennial swellings on savin-type junipers, and its aecia on pear leaves form squat, blunt, cage-like or trellised structures, not slender fringed tubes.

Finally, do not confuse rust aecia with insect galls or with the leaf spots of unrelated fungi. Insect galls are solid tissue enclosing a chamber and a grub; unrelated leaf-spot fungi produce dark pinhead fruit bodies flush with the leaf surface. Only the rust builds erect, orange-filled tubes with split, curling walls on the underside of a bright orange spot.