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Pathogen Biology

Rust fungi are obligate parasites. In nature, they require living host tissue for growth and reproduction; they cannot exist as saprophytes. In the absence of living host tissue, they survive as spores. In most rust fungi, only the teliospores are adapted to survive apart from a living host plant for more than a few months under field conditions.

Puccinia graminis is heteroecious. This word describes rust fungi that require two unrelated host plants, such as wheat and barberry, to complete their life cycle. Puccinia graminis is macrocyclic, producing all five spore stages: basidiospores, pycniospores (spermatia), aeciospores, urediniospores (uredospores), and teliospores. Anton deBary, in 1865, first recognized the nature of the heteroecious life cycle, but the role of each spore stage was not completely understood until John Craigie, a Canadian scientist, studied the pathogen in 1927.

Although stem rust is caused by a single species of fungus, Puccinia graminis, there is considerable genetic variation within the species. In 1884, Eriksson discovered host-specific subspecies or "special forms" of the fungus. Each special form is designated in Latin as a forma specialis or "f. sp." All of the formae speciales have an identical appearance, but vary in host range. The pathogen that causes stem rust of wheat (Triticum aestivum) is Puccinia graminis f. sp. tritici. Other formae speciales include P. graminis f.sp. secalis, causal agent of stem rust of rye (Secale cereale), and P. graminis f.sp. avenae, causal agent of stem rust of oat (Avena sativa). Both Puccinia graminis f. sp. tritici and P. graminis f.sp. secalis cause stem rust in barley. About 1916, E.C. Stakman and others determined that within P. graminis f.sp. tritici are further genetic subdivisions called races. Later, races were found within other formae speciales as well.

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