Anther Smut

Anther smut is caused by Microbotryum violaceum in the broad sense, a complex of closely related basidiomycete smuts in the Microbotryales. Modern work splits it into many host-specific species, notably Microbotryum lychnidis-dioicae on white campion (Silene latifolia) and M. silenes-dioicae on red campion (Silene dioica), but the fungus is still widely known collectively as anther smut.

The fungus is remarkable for what it does to the flower rather than to the leaves or stem. It grows systemically through the plant and, when flowering begins, colonises the developing anthers, replacing the pollen with its own violet, purple-brown or dull lilac teliospores. In female flowers of dioecious hosts it goes further, suppressing the ovary and inducing the development of sterile stamens that then carry spores, so a female plant produces spore-bearing anthers it would never otherwise have.

Because the spores sit exactly where pollen belongs, the fungus is transported by the same bees, flies, moths and butterflies that visit the flowers, making it one of the clearest natural examples of a pollinator-vectored fungus.

Flowers

The key sign is a flower that opens normally in size, shape and petal colour but whose anthers are loaded with dark violet to purplish-brown dust instead of pale yellow or cream pollen. A finger or the tip of a lens brushed across the anthers is stained purple-brown. On a plant that is otherwise ordinary in habit, a single glance into the flower is diagnostic.

Whole plant

Colonised plants are systemically occupied, so once one flower shows spores nearly all subsequent flowers on that plant do too, season after season for perennial hosts. Plants may be slightly shorter, with more numerous but smaller flowers, and female plants of dioecious species show the striking anomaly of developed stamens with an aborted ovary and shortened styles.

Spores

No spore print applies. Teliospores are globose to subglobose, roughly 6-10 micrometres across, violet-brown, and covered with a fine net-like reticulation of shallow polygonal meshes visible under high magnification. In mass they look distinctly violet rather than black or brown, which separates them from most grass smuts.

The fungus occurs on members of the pink family (Caryophyllaceae), especially Silene and Lychnis, with records also from Dianthus, Saponaria and Stellaria. It is found in meadows, hedge banks, roadsides, woodland rides, coastal cliffs, alpine grassland and gardens, wherever the host plants grow.

It is widespread across Europe and temperate Asia and has been introduced with its hosts to North America, where it is common in white campion populations. Spore-bearing flowers can be found through the whole flowering season, from late spring into autumn in mild regions, and the fungus overwinters inside the perennial crown or rosette of the host.

Microbotryum is an obligate biotroph and a systemic, sterilising parasite. Rather than killing its host, it converts the plant into a long-lived producer of fungal spores by hijacking the reproductive organs, so an infected plant can persist for years while producing no seed at all.

Spores deposited in an uncolonised flower germinate, undergo meiosis to produce haploid sporidia which mate, and the resulting dikaryotic hypha penetrates the flower or a young shoot and grows into the meristem. From there the fungus tracks the plant's growth and reaches every new flower. Because transmission depends on insect visitation, the system behaves much like a pollinator-transmitted parasite and has become a classic model in evolutionary biology for studying host specialisation, mating type genetics and sex chromosome evolution in fungi.

Ordinary dark-anthered flowers can cause momentary confusion, but true pollen is granular and yellow, cream or occasionally blue-grey, and it does not smear as a fine violet-brown powder. Sooty mould on flowers sits as a superficial black film on petals rather than filling the anther sacs.

Other smuts on Caryophyllaceae include Microbotryum dianthorum on pinks and carnations and various species occupying the ovaries instead of the anthers, such as smuts that replace seeds with spore masses inside the capsule; opening the capsule distinguishes them. Grass and cereal smuts like Ustilago produce black rather than violet spore masses and occur on entirely different hosts. In dioecious campions, the presence of well-formed stamens on a plant with an aborted ovary is a particularly strong indicator of this fungus.