Stem rust
From Wikipedia, the free encyclopedia
| Stem Rust | ||||||||||||||||
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| Puccinia graminis Pers. |
The stem, black or cereal rusts are caused by the fungus Puccinia graminis and are a significant disease affecting cereal crops.
[edit] Biology
There is considerable genetic diversity within the species P. graminis and several special forms, forma specialis, which vary in host range have been identified.
- Puccinia graminis f. sp. avenae, oat
- Puccinia graminis f. sp. dactylis
- Puccinia graminis f. sp. lolii
- Puccinia graminis f. sp. poae
- Puccinia graminis f. sp. secalis, rye, barley
- Puccinia graminis f. sp. tritici, wheat, barley
Like other Puccinia species, P. graminis has a complex life cycle featuring alternation of generations, the fungus is also heteroecious which means that its various life cycle stages require alternate host species. The complete life cycle of P. graminis requires barberry as well as a cereal species.
Stem rust infections on wheat plants convert from producing urediniospores to production of a specialized spore known as the teliospore. Teliospores have two nuclei each with a single set of chromosomes, there are two mating types + and -, which fuse to form a diploid cell that then undergoes meiosis to produce four haploid cells. Teliospores left in the field following the crop remain dormant until the next spring when they germinate and produce a type of spore known as a basidiospore. Each four celled teliospore gives rise to four basidiospores which undergo mitosis to produce the mature basidiospore which contains two haploid nuclei. Basidiospores cannot infect cereal plants. Instead, they infect young leaves of common barberry (Berberis vulgaris) or other susceptible Berberis, Mahonia, or Mahoberberis species or cultivars. On barberry, the resulting infections produce specialized infection structures called pycnia, which play an essential role in the sexual stage of the fungus.
Pycnia, which result from infection on young barberry leaves by basidiospores, are the sexual stage of the fungus life cycle. When a pycnium has been fertilized by pycniospores from a mating type compatible pycnium, its haploid cells become dikaryotic. The fertilized structure becomes an aecium, which produces chains of aeciospores surrounded by a cup-like enclosure of fungal cells. Like the urediniospores and like the cells of the aecium, each aeciospore contains two nuclei. And like the urediniospores, aeciospores infect wheat, not barberry.
[edit] Pathology
The stem rust fungus attacks the aboveground parts of the plant, infected plant produce fewer tillers and set fewer seed. In the case of extremely bad infection the plant may die. The site of infection is a visible symptom of the disease. Where infetion has occurred on the stem or leaf, elliptical blisters or pustules called uredia develop
Pycnia typically form on the upper side of barberry leaves, and aecia form within 5-7 days after fertilization on the lower side of the leaf directly below each fertilized pycnium.
[edit] References
- Schumann, G.L. and K.J. Leonard. 2000. Stem rust of wheat (black rust). The Plant Health Instructor. DOI: 10.1094/PHI-I-2000-0721-01
- Leonard, K.J. Black stem rust biology and threat to wheat growers, USDA ARS
