The south-facing sandstone cliffs of the Cumberland Plateau soak up heat all summer, then bathe the early winter forest in emitted warmth. Trees close to the cliffs keep their leaves for weeks after others have turned bare. Joseph Bordley and I took a walk along the cliffs yesterday to see what might be stirring. One of our finds were these round-leaved catchflies, Silene rotundifolia, growing out of cracks in the rock face. They’re still in bloom, although their pollinators (mostly hummingbirds) are long gone. A phenological time warp in the cliffs’ bubble of heat?

These plants might have caught more than flies. The anthers seem too fuzzy and purple for normal pollen. I’d welcome insight from botanists: does this look like normal Silene to you? If this is an infection, anther-smut fungus is the culprit.

The smut fungus infects cuts off the normal process of pollen production and converts the anthers to fungus spore factories. The spores are purple, hence the species name Microbotryum violaceum. The ovaries are also destroyed or damaged, so the plant is sterilized by the infection.

Microbotryum smut fungus is a sexually-transmitted disease, the gonorrhea of flowers. As bees, flies, or hummingbirds move among flowers they carry with them not just pollen, but unwanted fungal colonists. The pollinators’ promiscuous feeding habits makes them ideal transmitters of disease, poxed Don Juans that plants cannot turn away.

A study of herbarium records found that incidence of the fungus has increased over the last century in two closely related species, S. virginica and S. caroliniana. In Europe, the fungus seems to have spread northward after the last ice age, following its host plants as they expanded their range. Whether that is also true in North America is not known.

Apparently, the disease has not been found in S. rotundifolia, so I’m particularly keen to hear from readers who can comment on whether or not the anthers in the photos above are infected.

Bibliography:

Antonovics, Janis, Michael E. Hood, Peter H. Thrall, Joseph Y. Abrams, and G. Michael Duthie. 2003. “Herbarium Studies on the Distribution of Anther-Smut Fungus (Microbotryum Violaceum) and Silene Species (Caryophyllaceae) in the Eastern United States.” American Journal of Botany 90 (10) (October 1): 1522–1531. doi:10.3732/ajb.90.10.1522.
Vercken, Elodie, Michael C. Fontaine, Pierre Gladieux, Michael E. Hood, Odile Jonot, and Tatiana Giraud. 2010. “Glacial Refugia in Pathogens: European Genetic Structure of Anther Smut Pathogens on Silene Latifolia and Silene Dioica.” PLoS Pathog 6 (12) (December 16): e1001229. doi:10.1371/journal.ppat.1001229.