Scaly Robigo, god of rust, spare Ceres' grain; let silky blades quiver on the soil's skin. Let growing crops be nourished by a friendly sky and stars, until they ripen for the scythe … Spare us, I pray keep scabrous hands from the harvest. Harm no crops. The power to harm is enough.
—Ovid. 8. IV.911ff. In Fasti, translated in 2000 by A. J. Boyle and R. D. Woodard. Penguin Classics. Via Grout, James. “Robigalia” in Encyclopaedia Romana (Archived from the original on 12 February 2025).
As long as people have cultivated cereals, they have been blighted by rusts. The prayer documented by Ovid in the first century shows how these fungi made their way into ancient Roman religion, personified in a deity who had the power to destroy the year's harvest. Dogs were sacrificed to Robigo in an attempt to protect cereals from his wrath.
In the 20th century, an entire plant species was sacrificed on the altar of Robigo. American Barberry Berberis canadensis is a host of the infamous Wheat Stem Rust Puccinia graminis. On wheat the rust forms orange asexual aeciospores, which can quickly cover and destroy a whole crop. On barberries Berberis spp. it completes the sexual stage of its life cycle. In 1918, the US government decided to eradicate American Barberry to reduce Wheat Stem Rust, and by a few decades later, several hundred million shrubs had been killed. American Barberry is thought to have been fully eradicated in at least four of the thirteen states where it was historically found, and is considered Vulnerable to extinction1.
Rusts aren't just crop pests, though. As we saw in the last chapter, they have complex ecological interactions and are fascinating organisms in their own right. What makes them most unusual amongst other fungi is their ludicrously complex life cycle.
It all starts with an infected hostplant. The first host of the rust is infected by a haploid form of the fungus: it has a single copy of the genome. This haploid stage producing spermogonia. These produce gametes, in order to cross-fertilise another haploid rust's spermogonia. This is the stage that is vital to ensure the rust's offspring are genetically diverse, and for this reason rusts have evolved strategies like pseudoflowers to ensure spermogonia are cross-fertilised. One rust that infects thistles (Puccinia punctiformis, right) produces strong-smelling volatiles to attract flies. The chemicals produced are the same as those in the flowers of its host, with the exception of indole, which adds a strong putrid or faecal note to the mix2.