stem rust

Puccinia graminis
Scientific classification
Kingdom:	Fungi
Phylum:	Basidiomycota
Class:	Pucciniomycetes
Subclass:	Incertae sedis
Order:	Pucciniales
Family:	Pucciniaceae
Genus:	Puccinia
Species:	P. graminis
Binomial name
Puccinia graminis Pers., (1794)
Synonyms
Dicaeoma anthistiriae Puccinia albigensis Puccinia anthistiriae Puccinia brizae-maximae Puccinia cerealis Puccinia elymina Puccinia favargeri Puccinia graminis f. macrospora Puccinia graminis f.sp. avenae Puccinia graminis f.sp. secalis Puccinia graminis f.sp. tritici Puccinia graminis subsp. major Puccinia graminis var. graminis Puccinia graminis var. stakmanii Puccinia graminis var. tritici Puccinia jubata Puccinia linearis Puccinia megalopotamica Puccinia secalis Puccinia vilis Trichobasis linearis

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 due to the large numbers of people dependent on wheat for sustenance. The strain was named after the country where it was identified (Uganda) and the year of its discovery (1999).^[1] It spread to Kenya, then Ethiopia, Sudan and Yemen, and is becoming more virulent as it spreads.^[1] Scientists are working on breeding strains of wheat that are resistant to UG99. However, wheat is grown in a broad range of environments. This means that breeding programs would have extensive work remaining to get resitance into regionally adapted germplasms even after resistance is identified. ^[1]

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.

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.

Life cycle of Puccinia graminis

Pathology

The stem rust fungus attacks the parts of the plant which are above ground. Spores that land on green wheat plants form a pustule that invades the outer layers of the stalk.^[1] The site of infection is a visible symptom of the disease. Where infection has occurred on the stem or leaf, elliptical blisters or pustules called uredia develop. Infected plants produce fewer tillers and set fewer seed, and in cases of severe infection the plant may die.

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).^[2] It is virulent to the great majority of wheat varieties.^[3] 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. ^[4] The blight was first noted in Uganda in 1999 and has spread throughout the highlands of East Africa. In January 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.^[5]

History

The fungal ancestors of stem rust have infected grasses for millions of years and wheat crops for as long as they have been grown.^[1] According to Jim Peterson, professor of wheat breeding and genetics at Oregon State University, "Stem rust destroyed more than 20% of U.S. wheat crops several times between 1917 and 1935, and losses reached 9% twice in the 1950s," with the last U.S. outbreak in 1962 destroying 5.2% of the crop.^[1]

Weaponization

The United States produced wheat stem rust and rye stem rust as part of its biological weapons program, which was active from 1943-1969.^[6] Wheat and rye stem rust was produced from 1951-1957 at Fort Detrick then shipped to Edgewood Arsenal where it was classified, dried and stored.^[7] Again, from 1962-1969 wheat stem rust spores were produced and shipped to Rocky Mountain Arsenal where they too were classified, dried and stored.^[7] These anti-agriculture biological agents were tested at least 31 times by the U.S. military.^[8]

See also

Fungi portal

Wikimedia Commons has media related to: Puccinia graminis

• List of wheat diseases
• Robigus
• M115 bomb - A US cluster bomb designed to spread Stem rust

Notes

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
• Index Fungorum
• USDA ARS Fungal Database