Anther Smut Identification Guide

A Fungus You Find by Finding a Plant

Anther Smut breaks the usual rules of fungus hunting. There is no cap, no stalk, no gills and no bracket. Microbotryum violaceum lives inside a flowering plant and reveals itself only when that plant flowers, so the search image is botanical first and mycological second.

The setting is a hedge bank, a roadside verge, a coastal dune, a hay meadow, an alpine turf or a garden border — anywhere the host plants grow. Those hosts are members of the pink family, Caryophyllaceae, above all the campions: red campion, white campion, bladder campion, and their relatives, plus pinks and carnations in the genus Dianthus and soapwort in Saponaria.

The first impression, once you are looking at the right plants, is of flowers that look dirty. Among clean pale blooms, some carry stamens loaded with a dark violet to purple-brown dust where creamy pollen should be. Brush one with a fingertip and it leaves a purple smudge.

Confirming the Host

Start with the plant, because host identity is the strongest single character available. Look for opposite leaves in pairs up the stem, a swollen or inflated tubular calyx, five petals that are notched or split at the tip, and flowers arranged in forked branching clusters. Red and white campion are dioecious, with separate male and female plants; bladder campion has a conspicuously balloon-like calyx netted with veins.

Modern classifications split the old broad concept of Microbotryum violaceum into many narrowly host-specific species — one associated with white and red campion, another with bladder campion, others with pinks, soapwort and further hosts. Because those segregates are largely distinguished by host plus fine spore characters, recording the plant precisely is the most valuable thing you can do in the field.

Reading the Infected Flower

Open a flower and look directly at the stamens.

In an uninfected male campion flower the anthers are pale cream to yellowish and shed granular yellow pollen. In an infected flower the anthers are swollen, dull, and dark, and instead of pollen they are packed with a dry, powdery mass of violet, purple-brown or reddish-violet spores. The filaments may be elongated so the loaded anthers are pushed to the mouth of the flower where visiting insects cannot miss them.

Then check the ovary. In infected flowers it is aborted, shrunken and non-functional, so infected plants set no seed even though they keep flowering.

The most striking sign appears on female plants of dioecious hosts. Normally a female campion flower has no functional stamens at all. When infected, it develops sterile stamens that carry the fungal spore mass — the flower is effectively masculinised so that the fungus has anthers to occupy. A female-structured flower bearing dark spore-filled anthers is as close to conclusive as field observation gets.

Whole-Plant Signs

Step back and assess the whole plant. Infection is systemic: the mycelium persists in the growing points, so once a shoot is infected every subsequent flower it produces is infected too. A diseased plant is therefore uniformly affected rather than spotted here and there, which distinguishes it from most leaf-spotting fungi.

Infected plants often differ in stature: they may be shorter, more branched, with narrower leaves, and they frequently flower earlier and produce more but smaller flowers than uninfected neighbours. On perennial hosts the infection persists from year to year, so the same clump can be checked in successive seasons.

Reading the Spore Mass

There is no spore print to take in the conventional sense, but the equivalent step is straightforward and is where the identification is finally sealed.

Hold a white card or a microscope slide under an infected flower and tap the anthers. A fine violet-brown dust falls. Under a hand lens the deposit is uniform, dry and powdery, quite unlike the sticky, clumping grains of true pollen.

Under a compound microscope at high magnification, mount the dust in water or lactic acid and examine the teliospores. They are globose to subglobose, roughly 7 to 10 micrometres across, faintly violet-tinged, and their surface is finely reticulate — covered in a delicate raised net of ridges. Both the spore diameter and the mesh size of that net are used to distinguish the host-specific segregate species, so a measured set of spore dimensions plus a firm host identification is essentially a complete record.

Season and Distribution

Look whenever the hosts are in flower. Red campion peaks in spring and early summer on shaded hedge banks and woodland edges, white campion runs through summer on disturbed and arable margins, bladder campion flowers through summer on calcareous grassland and dunes, and garden and alpine pinks flower from late spring onward. The fungus is widespread throughout Europe, temperate Asia and North America wherever its hosts occur, including in introduced populations.

Ecology

The transmission route is the reason the fungus specialises in anthers. Bees, flies, moths and other pollinators visit infected flowers exactly as they visit uninfected ones, pick up spores on their bodies, and deposit them on the next flower they enter. The fungus then germinates, mates, and grows into the plant, spreading systemically to the meristem. It is a pollinator-transmitted infection that exploits the plant's own reproductive advertising, and it sterilises the host without killing it.

Separating It From Look-Alikes

Other anther-inhabiting smuts exist. Species of Antherospora occupy the anthers of bluebells, squills and grape hyacinths — different host families entirely, with paler or differently ornamented spores. Species of Urocystis and Thecaphora produce spore balls made of several spores stuck together rather than single free spores, visible immediately under a microscope.

Rust fungi on the same campions produce orange to brown pustules on leaves and stems, breaking through the epidermis, and never confine themselves neatly to anthers.

Naturally dark anthers occur in some cultivated pinks and other garden plants. Check with a lens: pollen grains are sticky, granular and adhere in clumps, while smut spores are dry, fine, and drift as dust.

Grey mould, Botrytis, on ageing flowers is grey to brownish, fuzzy and mycelial rather than a violet powder confined inside anthers.

Sooty moulds form superficial black films on leaf surfaces beneath aphid colonies and wipe off, and are never restricted to reproductive parts.

Field Checklist

A campion or pink; flowers with swollen anthers full of dry violet-brown powder in place of pollen; aborted ovaries; on dioecious hosts, female flowers bearing spore-laden stamens; systemic infection affecting every flower on the shoot; globose, finely reticulate spores 7 to 10 micrometres across.