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, Asia and most recently into Middle East and is causing major concern and an increase of food riots and civil unrest, notably in West and Central Africa among the worst-hit countries.
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.
In the spring and summer, stem rust infections on cereal plants produce dikaryotic urediniospores, which are spread by the wind to nearby cereal plants, where they germinate and infect cereals by penetrating through the stomata. This polycyclic asexual phase can rapidly spread the infection over a wide area. Towards the end of the growing season, the rust converts to producing teliospores, which again contain these two haploid nuclei of opposite mating types. Before the winter, the nucleii fuse to form a diploid cell, which remains dormant until the next spring when it undergoes meiosis to produce four haploid cells known as basidiospores, borne on a structure called a basidium. The basidiospores then undergo a mitotic nuclear division to produce the mature basidiospore which contains two haploid nuclei of the same mating type. Basidiospores cannot infect cereal plants, but are instead, carried in the wind, and infect young leaves of common barberry (Berberis vulgaris) or other susceptible Berberis, Mahonia, or Mahoberberis species or cultivars. On barberry, the basidiospore penetrates the leaf epidermis directly, and the resulting infections produce specialized infection structures called pycnia (or spermagonia).
Pycnia (or spermagonia), which result from infection on young barberry leaves by basidiospores, are the sexual stage of the fungus life cycle. When a receptive hypha from one pycnium has been fertilized by pycniospores (or spermatia) from a mating type compatible pycnium, its haploid cells become dikaryotic. The fertilized hypha forms an aecium, on the underside of the barberry leaf, which produces chains of aeciospores surrounded by a bell-like enclosure of fungal cells. Like the urediniospores and like the cells of the aecium, each aeciospore contains two nuclei. Aeciospores are carried by the wind, and infect cereals by penetrating through stomata. After infecting a cereal plant, the aecispores develop and form uredia under the plants epidermis, these produce the dikaryotic urediniospores. These uredia eventually rupture the plant's epidermis and again spread by the wind to nearby cereal plants, continuing the lifecycle.
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, 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. Unlike other rusts, which only partially affect crop yields, UG99 can bring 100% crop loss. Up to 80% yield losses were recently recorded in Kenya. 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. In March 2007, FAO announced its concern regarding the spread through Iran based on Iranian authorities report.