Azalea Leaf Gall Identification Guide

What You Are Actually Looking For

This is not a mushroom in the ordinary sense, and if you go looking for a cap and a stalk you will never find it. Exobasidium vaccinii lives inside the leaf and flower tissue of azaleas and their relatives and makes the plant build its fruiting structure for it. The swollen gall you find is transformed host tissue, and the fungus spreads its spore-bearing layer directly over the gall's outer surface. Once you accept that the "fruit body" is a modified leaf, the identification becomes straightforward.

Setting and Season

Spring into early summer, in cool damp weather, on the current season's soft new growth. The typical setting is a shaded, humid planting: azalea beds under trees, rhododendron borders, sheltered courtyards, nursery blocks, and hedges where air movement is poor. In the wild the same fungus and its close relatives turn up on Vaccinium — blueberry, bilberry, cranberry — in bogs, heaths and open woodland.

Search the newest leaves and flower parts, and look inside the bush as well as over its outer surface, because shaded interior growth is often the most heavily affected.

First Impression

A leaf that has gone wrong. Instead of a flat green blade there is a thickened, fleshy, bladder-like swelling. It may be a raised blister occupying part of one leaf, a whole leaf converted into a rounded succulent mass the size of a marble or a small plum, or a distorted flower bud swollen into a lumpy rosette. On a shrub carrying several, they stand out as pale, waxy shapes against the ordinary foliage.

Reading the Gall, Step by Step

Colour and stage. Galls pass through a predictable sequence, and knowing where yours sits in it prevents confusion. Young galls are pale green, often flushed pink, rose or dull red on the side facing the light. As the fungus matures, the surface pales toward ivory or waxy white. Then comes the diagnostic stage: a chalky, powdery white bloom spreads over the whole gall. That bloom is the fertile layer — a felt of basidia pushing out through the leaf's stomata and shedding spores — and it rubs off on a fingertip. Finally the gall dries, turns brown to dark brown, hardens, and may hang on the shrub as a shrivelled mummy for months.

Texture. Firm and succulent, snapping crisply like a thick pea pod. It should not be spongy with a hollow centre, resinous, corky, or woody.

Internal structure. Cut one open cleanly. A cross-section shows uniformly thickened, pale, watery tissue built of enlarged cells, sometimes with a hint of the original leaf's midrib running through it. There is no larval chamber, no insect, no seed-like core, no fungal cavity — just swollen plant tissue throughout. This single cut disposes of most of the look-alikes at once.

Continuity with the leaf. Trace the gall's outline back to the shoot. It is continuous with the leaf blade or the petiole, not stuck onto it, and you can often see the original leaf margin or midrib drawn into the swelling. Something that detaches cleanly and leaves a scar is a different phenomenon.

Surface under a lens. Before the bloom develops, the surface is smooth and slightly glossy, sometimes finely wrinkled. At bloom stage a hand lens shows a short, even, velvety white pile rather than the tangled cobwebby threads of a mould.

Collecting a Spore Deposit

You cannot make a conventional print, but you can collect spores and confirm the group. Take a gall at full bloom stage, hold it over a sheet of dark paper or a glass microscope slide, and tap it sharply; alternatively leave it overnight in a covered container resting above dark paper. The deposit is white and, against black, obvious.

Under a microscope the spores are hyaline and thin-walled, elongate-ellipsoid to slightly curved or sausage-shaped, and they commonly develop one to several cross-walls as they age. Many of them germinate where they lie and bud off small secondary spores, which is an attractive confirmation of the genus and something you can watch in a drop of water on a slide over a few hours.

Host and Ecology

The fungus is restricted to the heath family, Ericaceae, and this host restriction does much of the identification work for you. It persists from year to year in bud scales and on shoot surfaces, and in spring, when rain keeps expanding leaf tissue wet for long periods, spores germinate and the fungus grows between the cells of the young leaf. It stimulates the host cells to enlarge and multiply, producing the characteristic hypertrophied gall, then emerges through the stomata to fruit on the outside.

The cycle is essentially once per growth flush, so a cool wet spring produces many galls and a warm dry one very few. Galls do not spread within a leaf after the tissue has hardened, which is why they are confined to the season's soft growth.

Separating It from Look-alikes

Other Exobasidium species. The genus contains many species, each with a narrow host range, and separation is mostly by host plant combined with gall form. E. rhododendri produces large red galls on alpine rhododendron; E. camelliae makes thick pale leaf galls on camellia; several other species cause pale leaf spots, shoot deformation or witches' broom rather than fleshy galls. Record the exact host plant.

Insect galls. The critical test is the cut. Midge, wasp and mite galls contain internal chambers, larvae, or a distinctly firm or woody core, and their surfaces are often hairy, pimpled or ribbed rather than smooth and waxy. They also tend to be attached to the leaf as a discrete structure rather than being the leaf itself.

Taphrina deformations. Analogous puckering and thickening of leaves, but on birch, alder, plum and peach — different plant families entirely — and produced by an ascomycete, whose bloom is a layer of spore sacs.

Powdery mildew. Produces a white bloom, but on flat, essentially undistorted leaves, and the coating is a web of surface threads rather than an even velvety pile on a swollen organ.

Botrytis blight. Grey-brown fuzzy growth on collapsing flowers and soft shoots; the tissue goes limp and brown rather than firm and swollen.

Leaf rust (Pucciniastrum and relatives). Orange to yellow powdery pustules, mainly on the leaf underside, with no thickening of the blade.

Oedema and physiological blistering. Small corky raised spots or water-soaked bumps on the leaf underside, caused by uneven water uptake, with no white bloom and no spores.

Field Checklist

Host in the heath family; current-season growth; a fleshy, firm, bladder-like swelling that is continuous with the leaf; pale green flushed pink, then ivory, then dusted with a chalky white bloom, then brown and shrivelled; solid pale tissue with no chamber or insect inside; a white spore deposit of curved, septate spores that bud in water; spring, in cool damp shade.