Chapter 22: Urediniomycetes: Rust Fungi (Uredinales)

Rust fungi are distinguished by their capacity to produce up to five morphologically distinct spore types including spermatia, aeciospores, urediniospores, teliospores, and basidiospores, frequently involving complex heteroecius cycles that alternate between two unrelated host plants, though autoecious and simplified life cycle variants also occur. The overwintering teliospores germinate to form transversely septate metabasidia that generate basidiospores for infecting alternate hosts, while dikaryotic mycelial tissues develop specialized haustorial structures that facilitate nutrient extraction while maintaining obligate biotrophy throughout the fungal lifecycle. Puccinia graminis represents the classical model organism, causing black stem rust on wheat with a multiphase infection cycle that depends on barberry as the alternate sexual host, encompassing spermogonial stages that produce nectar for insect attraction, aecial stages generating distinctive orange spores, uredial stages producing repeating pustules that drive epidemic spread through urediniospore dispersal, and telial stages forming black overwintering structures. Historical epidemics of P. graminis caused catastrophic crop losses exceeding fifty percent, with pathogen populations dispersing northward annually through the Puccinia pathway from southern United States to Canadian regions. Barberry eradication initiatives successfully reduced new virulence race formation by limiting sexual recombination opportunities, though pathogenic diversity persists through mutation and somatic hybridization mechanisms. The genus Puccinia encompasses over four thousand species including formae speciales specialized on different cereal crops, while additional major cereal rusts include P. triticina on wheat with chocolate-brown lesions, P. striiformis adapted to cooler climates with yellow uredinia, P. hordei on barley, and P. coronata on oats featuring distinctively spiny teliospores. Beyond cereals, Hemileia vastatrix causes coffee leaf rust representing the world's most economically significant coffee disease, Cronartium species produce pine blister rust alternating with currant and oak hosts, Gymnosporangium alternates between junipers and Rosaceae hosts, and Melampsora species infect poplars and willows, with these latter pathogens proving instrumental in establishing foundational concepts in host-pathogen genetic interactions.