Stripe rust Puccinia striiformis on wheat leaf, copyright International Maize and Wheat Improvement Center.

Belongs within: Pucciniaceae.

Puccinia is a diverse genus of rust fungi infecting angiosperms. Species may be heteroecious or autoecious, macro- or microcyclic; teliospores are usually two-celled, more rarely one-celled or composed of several cells separated by transverse septa (Kirk et al. 2001). Some species are significant economic pests such as the stem rust P. graminis and stripe rust P. striiformis.

Rust, anyone?
Published 13 March 2018

At certain times of year, when the weather is warm, you may see patches of yellow or orange appear on plant leaves. It is often particularly notable on grass. These patches are known as rust and are the fruiting bodies of parasitic fungi. In some cases, they may be merely a nuisance or an eyesore. In other cases, their effects can be devastating. Rust fungi may cause enormous damage to commercial crops. One particularly nasty strain of the stem rust Puccinia graminis that goes by the label of TTKSK or Ug99 has been spreading through Africa and Asia since its discovery in Uganda in 1999, causing up to 100% losses in wheat crops where it hits. A similar strain of the same species was recently involved in outbreaks in southern Europe. And this rust can’t just be covered over with a bit of bog.

Stem rust Puccinia graminis uredia on wheat, from the US Dept of Agriculture.

Puccinia is the largest genus of rusts with around 3000 known species (Liu & Hambleton 2010), infecting a wide range of host plants. Many rusts have complicated life cycles…or perhaps that should be ‘insane’. Some of you may be aware that, until recently, mycologists (researchers of fungi) maintained a system of dual nomenclature that classified sexual and asexual forms of fungi separately, due to the difficulty in matching one to the other*. Rust fungi can have a life cycle involving a sexually reproducing stage and two different asexually reproducing stages on two different hosts, all of them distinct in appearance, so many rust fungal species could masquerade under no less than three distinct names! But then, some species might have simpler life cycles dropping one or more of the possible stages, and some might restrict their attentions to a single host. The difficulty of wrapping one’s head around rust life cycles may perhaps best be conveyed by reproducing one paragraph from the review by Petersen (1974), which I invite you to look upon below in all its hideous hideousness:

*I believe that the botanical code of nomenclature was recently changed to no longer allow this set-up as a formal system, but I presume that it’s going to take a long time to work that one through.

A complex system of nomenclature has been developed to quickly indicate the stages found in any particular life cycle in the rusts. While easily understood by students of the group with some experience, the system at first appears bewildering. Those taxa which exhibit all five stages during their life history are called Euforms. They may be Heter-Eu- (infecting more than one host) or Aut-Eu- (occurring on a single host). In some rusts, the aecial stage is deleted, or the aecia and aeciospores are morphologically identical to uredia and uredospores, these organisms being termed Brachy-forms. All these forms are autoecious, thus enabling the “aut-” prefix to be dropped. For those organisms in which spermogonia and spermatia are missing, Maire used Cata- as a prefix, but this usage is rarely seen nowadays. When the uredial stage has been dropped, the organism is called an Opsis-form. This may be used as a prefix, such as Opsis- Gymnosporangium, or more commonly as a suffix, such as Gymno- sporangiopsis. Again, forms can be Heter-Opsis-, or Aut-Opsis-. If this life cycle also deleted spermogonia, it was dubbed Catopsis- by Maire. In more general terminology, rust fungi exhibiting chiefly teliospores (with or without spermogonia) are known as Micro-forms, but Maire again specified those which exhibited both telia and spermogonia as Hypo-forms. In these forms, the teliospores are normal in that they require a resting period before germination. In some taxa, teliospore-like propagules are produced which are lighter in color, exhibit thinner walls, and more obscure germ pores, and which require no resting period before germination, often germinat- ing in situ. These spores have been called leptospores, and the life cycle, otherwise identical to that of Micro-forms, is known as Lepto- form. Occasionally, only uredospores and teliospores are found (these sometimes are thought of as imperfect rusts in which other stages will hopefully be found), and these are called Hemi-forms. Finally, in some taxa the teliospores are cytologically similar to aeciospores, in which case the life cycle is called Endo-, the species with such structures often segregated in the genus Endophyllum.

Life cycle of stem rust Puccinia graminis, from US Dept of Agriculture.

A typical ‘full’ rust life cycle is the one gone through by stem rust, shown in the diagram above. Stem rust alternates between two hosts, grasses such as wheat (it also infects related species such as barley or rye) and barberry. Sexual reproduction occurs on barberry near the beginning of the growing season when haploid spores known as spermatia or pycniospores are produced from fruiting bodies called spermatogonia or pycnia. In Puccinia species, these spermatogonia are flask-shaped and tend to be evenly spaced across the host tissue; other rust fungi may produce more irregular and irregularly-spaced spermatogonia. At the sides of the flask’s opening are protruding hyphae to which spermatia from other spermatogonia fuse. Now, in animals such as ourselves, fusion of sperm and ovum is usually immediately followed by fusion of their respective haploid nuclei to form the diploid daughter nucleus. In rusts, however, the haploid parent cells fuse but their nuclei do not. Instead, the daughter cell grows and divides as a dikaryotic organism with two nuclear lineages remaining associated but distinct in each cell. The dikaryotic mycelium produced from fusion of spermatium and receptive hypha gives rise to a fruiting body known as an aecium which in Puccinia is cup-shaped. The aecium produces its own spores that are shed to infect the alternate host, the grass, to which they gain access through the host’s stomata. Germinating aeciospores grow into a mycelium that penetrates host cells, absorbing nutrients directly from the host cytoplasm. When the time comes for the next reproductive stage, the rust produces a compacted layer called a uredinium that gives rise to yet another spore type, urediniospores. Unlike aeciospores that travel from one host to another, urediniospores are able to re-infect the same host, giving rise to a new uredinial stage of the life cycle. This asexual sub-cycle continues indefinitely for as long as growing conditions remain good for the rust. When conditions deteriorate, the rust stops producing urediniospores and begins producing thick-walled teliospores that are able to persist through the cold winter. It is within the teliospores that the dikaryotic nuclei finally fuse, giving rise to daughter nuclei that then themselves undergo meiosis so the teliospore germinates at the beginning of the next season to release haploid basidiospores, that infect a barberry to begin the cycle anew.

Arum rust Puccinia sessilis aecia on leaf of Arum maculatum, copyright Velella.

Because of the need for two hosts in the life cycle, crop pests such as stem rust may potentially be controlled by eradicating the second host. However, first you have to know what to target. Stripe rust Puccinia striiformis is another significant pest of grass crops whose alternate host was not identified as barberry until 2010 (Jin et al. 2010). And in warmer climates where urediniospores can survive all year round, rusts may be able to persist asexually even without a suitable alternate host.

Systematics of Puccinia
Puccinia Pers. 1801 (see below for synonymy)KC01
|--P. antirrhiniKC01
|--P. arachidisKC01
|--P. arrhenateriL98
|--P. bougainvilleae Schröter in Henn. 1896 [incl. Aecidium bougainvilleae Spegazzini 1881]HH03
|--P. calcitrapaeBG96
| |--P. c. var. calcitrapaeBG96
| `--P. c. var. centaureaeBG96
|--P. canaliculataKC01
|--P. cancellata Sacc. & Roum. 1881 [incl. Uredo cancellata Dur. & Mont. 1846]BG96
|--P. carthami Corda 1840 [=Bullaria carthami (Corda) Arth. & Mains 1922]BG96
|--P. cenchriBG96
| |--P. c. var. cenchriBG96
| `--P. c. var. africana Cumm. 1952 [incl. Uredo cenchricola Henn. 1908]BG96
|--P. chondrillinaW91
|--P. chryanthemiKC01
|--P. ciliataHH03
|--P. cordiae Arthur 1916 [=Bullaria cordiae (Arthur) Arthur & Mains in Arthur 1921; incl. Uredo cordiae Henn. 1904]HH03
|--P. coronataKD83
|--P. corticolaHH03
|--P. cynodontis Lacroix ex Desm. 1859 [incl. Uredo eleusine-indicae Saw. 1943, Aecidium plantaginis]BG96
|--P. cyperi-tegetiformis Kern 1919 [=Uredo cyperi-tegetiformis Henn. 1905]BG96
|--P. cyrnaea [incl. P. rimosa]BG96
|--P. distinctaC08
|--P. fragosoana Beltrán 1921 [incl. Uredo schizachyrii Doidge 1928]BG96
|--P. graminis Pers. 1801BG96
| | i. s.: P. g. var. triticiD51
| |--P. g. ssp. graminis [incl. P. albigensis Mayor 1957, Aecidium berberidis]BG96
| `--P. g. ssp. graminicolaBG96
|--P. helianthiKC01
|--P. heterosporaKC01
|--P. hieraciiM97
|--P. holosericeaDP72
|--P. hordeiKC01
|--P. horianaKC01
|--P. hypochaeridisC08
|--P. imperatae Poirault 1913 [=Uredo imperatae Magn. 1900]BG96
|--P. isiacae Wint. in Kuntze 1887BG96
|--P. johnstoniiHH03
|--P. kuehniiKC01
|--P. lagenophorae Cooke 1884 [incl. P. terrieriana Mayor 1962]BG96
|--‘Aecidium’ magellanicumHH03
|--P. magnusiana Koern. 1876 [incl. P. alnetorum Gaeum. 1941]BG96
|--P. malvacearumKC01
|--P. melanocephataKC01
|--P. menthae Pers. 1801BG96
|--P. ornata Arthur & Holw. in Arthur 1887HH03
|--P. pampeana Spegazzini 1880 (see below for synonymy)HH03
|--P. perplexansKC01
| |--P. p. f.sp. perplexansKC01
| `--P. p. f.sp. triticinaKC01
|--P. polygoni-amphibii Pers. 1801 [incl. Aecidium sanguinolentum Lindr. 1900]BG96
|--P. polypogonis Speg. 1909 [=Uredo polypogonis Speg. 1899]BG96
|--P. polysoraKC01
|--P. punctiformisKC01
|--P. purpureaP99
|--P. recondita Roberge ex Desmazieres 1857BG96 [incl. P. dasypyri Guyot & Malen. 1963BG96, P. dispersaKC01]
|--P. scirpi DC. 1805BG96
|--P. sorghiKD83
|--P. striiformis Westend. 1854BG96
| |--P. s. var. striiformisBG96 (see below for synonymy)
| `--P. s. var. dactylidisBG96
|--P. stylidiiW05
|--P. suaveolensKD83
`--P. tuyutensis Speg. 1881 [incl. Aecidium cressae DC. 1815, P. cressae Lag. 1889]BG96

Puccinia Pers. 1801 [incl. Allodus Arthur 1906, Argomyces Arthur 1912, Argotelium Arthur 1906, Bullaria DC. 1805, Coronotelium Syd. 1921, Cutomyces Thüm. 1878, Dicaeoma Gray 1821, Eriosporangium Bertero ex Ruschenb. 1831, Hypodermium Link 1816, Jackya Bubák 1902, Leptinia Juel 1897, Leptopuccinia (Winter) Rostr. 1902, Lindrothia Syd. 1922, Linkiella Syd. 1921, Lysospora Arthur 1906, Micropuccinia Rostr. 1902, Persooniella Syd. 1922, Pleomeris Syd. 1921, Poliomella Syd. 1922, Puccinidia Mayr 1890, Rostrupia Lagerh. 1889, Schroeterella Syd. 1922 non Herzog 1916, Sclerotelium Syd. 1921, Solenodonta Castagne 1845, Trailia Syd. 1922 non Sutherland 1915]KC01

Puccinia pampeana Spegazzini 1880 [=Dicaeoma pampeana (Spegazzini) Kuntze 1898; incl. P. araucana Dietel & Neger 1897, Aecidium capsici Kern & Whetzel 1930, P. capsici Mayor 1913, P. capsicicola Kern & Thurst. 1940 (n. n.), P. gonzalezii Mayoir 1913, Aecidium solaninum var. laevis Spegazzini 1909, A. pampeanum Spegazzini 1880, Endophyllum pampeanum (Spegazzini) Lindq. 1963, Puccinia paulensis Rangel 1918, Aecidium solaninum Spegazzini 1881, P. solanina Spegazzini 1912]HH03

Puccinia striiformis Westend. 1854 var. striiformisBG96 [incl. Uredo glumarum Schmidt 1827BG96, P. glumarumKC01, P. stapfiolae Mundk. & Thirum. 1946BG96]

*Type species of generic name indicated


[BG96] Baka, Z. A., & H. B. Gjaerum. 1996. Egyptian Uredinales. 1. Rusts on wild plants from the Nile Delta. Mycotaxon 60: 291–303.

[C08] Cheel. 1908. Notes and exhibits. Proceedings of the Linnean Society of New South Wales 33: 736.

[DP72] Deighton, F. C., & K. A. Pirozynski. 1972. Microfungi. V. More hyperparasitic hyphomycetes. Mycological Papers 128: 1–110.

[D51] Dobzhansky, T. 1951. Genetics and the Origin of Species 3rd ed. Columbia University Press: New York.

[HH03] Hernández, J. R., & J. F. Hennen. 2003. Rust fungi causing galls, witches’ brooms, and other abnormal plant growths in northwestern Argentina. Mycologia 95 (4): 728–755.

Jin, Y., L. J. Szabo & M. Carson. 2010. Century-old mystery of Puccinia striiformis life history solved with the identification of Berberis as an alternative host. Phytopathology 100: 432–435.

[KC01] Kirk, P. M., P. F. Cannon, J. C. David & J. A. Stalpers. 2001. Ainsworth & Bisby’s Dictionary of the Fungi 9th ed. CAB International: Wallingford (UK).

[KD83] Koç, N. K., & G. Défago. 1983. Studies on the host range of the hyperparasite Aphanocladium album. Phytopathologische Zeitschrift 107: 214–218.

[L98] Lienhard, C. 1998. Faune de France. France et Régions Limitrophes. 83. Psocoptères Euro-Méditerranéens. Fédération Française des Sociétés de Sciences Naturelles: Paris.

Liu, M., & S. Hambleton. 2010. Taxonomic study of stipe rust, Puccinia striiformis sensu lato, based on molecular and morphological evidence. Fungal Biology 114: 881–899.

[M97] McAlpine, D. 1897. Two additions to the fungi of New South Wales. Proceedings of the Linnean Society of New South Wales 21 (4): 722–724, pl. 56.

Petersen, R. H. 1974. The rust fungus life cycle. Botanical Review 40 (4): 453–513.

[P99] Pinto, N. F. J. de A. 1999. Avaliação de fungicidas no controle de Sphacelia sorghi (Claviceps africana) agente etiológico da “ergot” ou doença açucarada do sorgo. Summa Phytopathologica 25: 4–8.

[W91] Waterhouse, D. F. 1991. Insects and humans in Australia. In: CSIRO. The Insects of Australia: A textbook for students and research workers 2nd ed. vol. 1 pp. 221–235. Melbourne University Press: Carlton (Victoria).

[W05] Wege, J. A. 2005. Stylidium validum (Stylidiaceae): a new trigger plant from southern Western Australia. Journal of the Royal Society of Western Australia 88 (1): 13–16.

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