Chapter 23: Ustilaginomycetes: Smut Fungi and Allies

Ustilaginomycetes comprise approximately 1500 species within the Basidiomycota, nearly all functioning as obligate plant pathogens with yeast-like saprotrophic phases. This class differs from rust fungi through their simpler intracellular haustoria and their production of numerous sporidia rather than the typical four basidiospores from basidia or promycelia. The true smut fungi within the subclass Ustilaginomycetidae cause economically significant smut diseases characterized by darkly pigmented sori containing teliospores that replace host reproductive tissues. These organisms exhibit complex life cycles alternating between haploid yeast-like sporidia and dikaryotic infectious hyphae, with mating systems varying from bipolar patterns in Ustilago hordei to tetrapolar systems in Ustilago maydis. Teliospore dispersal occurs primarily through wind and seeds, with some species dispersed by insects, and many teliospores remain viable in soil or on seeds for extended periods. Major cereal pathogens include covered smut of barley, oat smut, loose smut of barley, and loose smut of wheat, all causing systemic infections in seedlings or floral tissues with prolonged latent phases before sori development replaces seeds at maturity. Corn smut, caused by Ustilago maydis, exhibits atypical behavior by infecting multiple maize organs and producing hypertrophied galls known as huitlacoche in Mexico, while simultaneously serving as a critical genetic model organism revealing signal transduction pathways, cytoskeletal dynamics, mating pheromone systems, and virulence mechanisms. The genus Tilletia encompasses bunt fungi infecting wheat and rice, with common bunt and dwarf bunt representing historically significant pathogens, though Karnal bunt caused international trade disruptions despite minimal yield impact. Control strategies include fungicidal seed treatments with carboxin, hot water seed disinfection, deployment of resistant cultivars, seed certification programs, and modern molecular detection using polymerase chain reaction. The Microbotryales, traditionally classified as smuts but reclassified as Urediniomycetes, includes anther smut, which replaces pollen with teliospores and induces sex reversal in infected plants. The Exobasidiales differ fundamentally by producing basidia directly from infected host tissues rather than from teliospores, with species parasitizing plants in the Ericaceae family and causing distinctive hypertrophy and discoloration of leaves and shoots. Together, these fungi represent major agricultural threats, economically important pathogens, and invaluable model systems for understanding fungal genetics, pathogenicity mechanisms, and plant-microbe interactions.