Stem rust

The stem, black or cereal rusts are caused by the fungus Puccinia graminis and are a significant disease affecting cereal crops. An epidemic of stem rust on wheat caused by race UG99 is currently spreading across Africa and into Asia and is causing major concern.

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.

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.

UG99
UG99, which has the designation of TTKS, is a race of black stem rust (Puccinia graminis tritici). It is virulent to the great majority of wheat varieties. The blight was first noted in Uganda in 1999 and has spread throughout the highlands of East Africa. In January of 2007, spores blew across to Yemen, and north into Sudan.