Stem rust
Encyclopedia
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). It spread to Kenya, then Ethiopia, Sudan and Yemen, and is becoming more virulent as it spreads. 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 resistance into regionally adapted germplasms even after resistance is identified.
, which vary in host range have been identified.
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 cereal
s 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 cereal
s 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.
Left: thin section through a Chara antheridium, note the layer of non-reproductive cells around the inner reproductive cells; right: spermatozoids in long strings of spermatozoid-producing cells from an antheridium.
of black stem rust (Puccinia graminis tritici).
It was discovered in Uganda, Africa in 1999 and has since spread to many other African countries. It is a race of black stem rust TTKS caused by the fungus Puccinia graminis tritici and is historically the most devastating fungal disease to wheat and barley that can result in 100% crop loss. This strain of wheat rust, located in Ugandan wheat fields, received its abbreviated name based on the country and year in which it was found. Seven races belonging to the Ug99 lineage are now known and have spread to various wheat-growing countries in the eastern African highlands, as well as Zimbabwe, South Africa, Sudan, Yemen, and Iran. While it is devastating South Africa and Middle Eastern wheat production and is expected to make its way into Pakistan and the Indian subcontinent, it has yet to enter the United States. This exclusive variety of wheat rust is devastating to the production of the crop because it kills 80-90% of global wheat cultivars under suitable conditions, according to the Food and Agricultural Organization of the United Nations. Worldwide, this strain of wheat stem rust is a threat to global wheat production threatening food securities in developing countries.
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 2007, spore
s 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.
The first stage, referred to as Stage 0, begins in the Spermogonium and is the sexual reproduction stage. The Spermogonia emanates sweet nectar that attracts flies while encouraging the spermatia and receptive hyphae to “meet”. When the nectar is released and the flies come to drink the liquid the spermatia attach to the fly and are placed on the hyphae by way of the fly, thus beginning sexual reproduction. This step in the cycle is very important, and if it does not take place, the next phase will not occur. Stage 1 takes place in the aeciospores of the aecia. These spores are binucleate and are formed in a chainlike series. They are born on barberry and are spread by the wind and are the spores that actually infect the wheat. Urediniospores in the uredia are the summer spores of the following stage, Stage 2, and are the source of the characteristic rust spots that arise. Urediniospores are the most damaging spore to the wheat and can infect other plants throughout the spring and summer months due to the spores being distributed by the wind. Stage 3, referred to as the resting stage, gives rise to teliospores, also called resting spores, in the telia. This is an overwintering stage, and when spring comes around, each teliospore is capable of germination of basidia and basidiospres. The final stage, Stage 4, is the transitional stage where the reproductive basidiospores in the basidia infect the barberry host.
, "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.
While Ug99 wasn’t discovered until 1999, stem rust has been an ongoing problem dating back to Aristotle’s time (384-322 B.C). An early ancient practice by the Romans was one where they would sacrifice red animals such as foxes, dogs, and cows to Robigo or Robigus, the rust god. They would perform this ritual in the spring during a festival known as the Robigalia in hopes of the wheat crop being spared from the destruction caused by the rust. Weather records from that time have been observed and it has been speculated that the fall of the Roman Empire was due to a string of rainy seasons in which the rust would have been more harsh, resulting in reduced wheat harvests. The first laws banning barberry were established in 1660 in Rouen, France. This was due to the fact that European farmers had correlated a relationship between barberry and stem rust epidemics in wheat. The law banned the planting of barberry near wheat fields and was the first of its kind before the parasitic nature of stem rust was discovered in the 1700’s.
Two Italian scientists Fontana and Tozzetti first explained the stem rust fungus in wheat in 1767. Thirty years later it received its name, Puccinia graminis, by Persoon and in 1854 the Tulasne brothers discovered the characteristic five-spore stage that is known to some stem rust species. The brothers were also able to make a connection between the red (urediniospore) and black (teliospore) spores as different stages within the same organism, but the rest of the stages remained unknown.
Anton de Bary later conducted experiments to observe the beliefs of the European farmers regarding the relationship between the rust and barberry plants and after successful attempts to connect the basidiospores of the basidia stage to barberry, he also identified that the aeciospores in the aecia stage reinfect the wheat host. Upon de Bary’s discovery of all five spore stages and their need for barberry as a host, John Craigie, a Canadian pathologist, identified the function of the spermogonium in 1927.
Due to the useful nature of both barberry and wheat plants, they were eventually brought to the United States by European colonists. Barberry was used for many things like making wine and jams from the berries to tool handles from the wood. Ultimately, as they did in Europe, the colonists began to notice a relationship between barberry and stem rust epidemics in wheat. Laws were enacted in many New England colonies, but as the farmers moved west, the problem with the stem rust moved with them and began to spread to many areas, creating a devastating epidemic in 1916. It wasn’t until two years later in 1918 that the United States created a program to remove barberry. The program was one that was supported by state and federal entities and was prompted by the looming fear of food supplies during the war. The “war against barberries” was waged and called upon the help of citizens through radio and newspaper advertisements, pamphlets, and fair booths asking for help from all in the attempt to rid the barberry bushes of their existence. Later, in 1975-1980, the program was reestablishes back to state jurisdiction. Once this happened, a federal quarantine was established against the sale of stem rust susceptible barberry in those states that were part of the program. A barberry testing program was created to ensure that only the species of barberry and other variations of plants that are immune to stem rust will be grown in the quarantine area.
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
Puccinia graminis and are a significant disease affecting cereal crops. An epidemic
Epidemic
In epidemiology, an epidemic , occurs when new cases of a certain disease, in a given human population, and during a given period, substantially exceed what is expected based on recent experience...
of stem rust on wheat
Wheat
Wheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...
caused by race Ug99 is currently spreading across Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
, Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
and most recently into Middle East
Middle East
The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far 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). It spread to Kenya, then Ethiopia, Sudan and Yemen, and is becoming more virulent as it spreads. 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 resistance into regionally adapted germplasms even after resistance is identified.
Biology
There is considerable genetic diversity within the species P. graminis and several special forms, forma specialisForma specialis
Forma specialis is a taxonomic grouping allowed by the International Code of Botanical Nomenclature, that is applied to a parasite which is adapted to a specific host ; this classification may be applied by authors who do not feel that a subspecies or variety name is...
, which vary in host range have been identified.
- Puccinia graminis f. sp. avenae, oatOatThe common oat is a species of cereal grain grown for its seed, which is known by the same name . While oats are suitable for human consumption as oatmeal and rolled oats, one of the most common uses is as livestock feed...
- Puccinia graminis f. sp. dactylis
- Puccinia graminis f. sp. lolii
- Puccinia graminis f. sp. poae
- Puccinia graminis f. sp. secalis, ryeRyeRye is a grass grown extensively as a grain and as a forage crop. It is a member of the wheat tribe and is closely related to barley and wheat. Rye grain is used for flour, rye bread, rye beer, some whiskeys, some vodkas, and animal fodder...
, barleyBarleyBarley is a major cereal grain, a member of the grass family. It serves as a major animal fodder, as a base malt for beer and certain distilled beverages, and as a component of various health foods... - Puccinia graminis f. sp. tritici, wheatWheatWheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...
, barleyBarleyBarley is a major cereal grain, a member of the grass family. It serves as a major animal fodder, as a base malt for beer and certain distilled beverages, and as a component of various health foods...
Like other Puccinia
Puccinia
Puccinia is a genus of fungi. All species in this genus are obligate plant pathogens and are known as rusts .Examples of Puccinia rusts and the diseases they cause:* Puccinia asparagi - Asparagus rust...
species, P. graminis has a complex life cycle featuring alternation of generations
Alternation of generations
Alternation of generations is a term primarily used in describing the life cycle of plants . A multicellular sporophyte, which is diploid with 2N paired chromosomes , alternates with a multicellular gametophyte, which is haploid with N unpaired chromosomes...
, the fungus is also heteroecious
Heteroecious
A heteroecious parasite is one that requires at least two hosts. The primary host is the host in which the parasite spends its adult life; the other is the secondary host. Both the primary host and an unrelated alternate host are required for the parasite to complete its life cycle...
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
Cereal
Cereals are grasses cultivated for the edible components of their grain , composed of the endosperm, germ, and bran...
species.
In the spring and summer, stem rust infections on cereal
Cereal
Cereals are grasses cultivated for the edible components of their grain , composed of the endosperm, germ, and bran...
plants produce dikaryotic urediniospores, which are spread by the wind to nearby cereal
Cereal
Cereals are grasses cultivated for the edible components of their grain , composed of the endosperm, germ, and bran...
plants, where they germinate and infect cereal
Cereal
Cereals are grasses cultivated for the edible components of their grain , composed of the endosperm, germ, and bran...
s by penetrating through the stomata. This polycyclic
Polycyclic
Polycyclic may refer to:* Polycyclic compound, in organic chemistry, a cyclic compound with more than one hydrocarbon loop or ring structures* Polycyclic group, in mathematics, a solvable group that satisfies the maximal condition on subgroups...
asexual
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
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
Karyogamy
Karyogamy is the fusion of pronuclei of two cells, as part of syngamy, fertilization, or true bacterial conjugation.It is one of the two major modes of reproduction in fungi...
to form a diploid cell, which remains dormant until the next spring when it undergoes meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
to produce four haploid cells known as basidiospores
Basidium
thumb|right|500px|Schematic showing a basidiomycete mushroom, gill structure, and spore-bearing basidia on the gill margins.A basidium is a microscopic, spore-producing structure found on the hymenophore of fruiting bodies of basidiomycete fungi. The presence of basidia is one of the main...
, borne on a structure called a basidium
Basidium
thumb|right|500px|Schematic showing a basidiomycete mushroom, gill structure, and spore-bearing basidia on the gill margins.A basidium is a microscopic, spore-producing structure found on the hymenophore of fruiting bodies of basidiomycete fungi. The presence of basidia is one of the main...
. The basidiospores
Basidium
thumb|right|500px|Schematic showing a basidiomycete mushroom, gill structure, and spore-bearing basidia on the gill margins.A basidium is a microscopic, spore-producing structure found on the hymenophore of fruiting bodies of basidiomycete fungi. The presence of basidia is one of the main...
then undergo a mitotic
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
nuclear division to produce the mature basidiospore
Basidium
thumb|right|500px|Schematic showing a basidiomycete mushroom, gill structure, and spore-bearing basidia on the gill margins.A basidium is a microscopic, spore-producing structure found on the hymenophore of fruiting bodies of basidiomycete fungi. The presence of basidia is one of the main...
which contains two haploid nuclei of the same mating type. Basidiospores
Basidium
thumb|right|500px|Schematic showing a basidiomycete mushroom, gill structure, and spore-bearing basidia on the gill margins.A basidium is a microscopic, spore-producing structure found on the hymenophore of fruiting bodies of basidiomycete fungi. The presence of basidia is one of the main...
cannot infect cereal
Cereal
Cereals are grasses cultivated for the edible components of their grain , composed of the endosperm, germ, and bran...
plants, but are instead, carried in the wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
, and infect young leaves of common barberry
Berberis vulgaris
Berberis vulgaris /// is a shrub in the family Berberidaceae, native to central and southern Europe, northwest Africa and western Asia; it is also naturalised in northern Europe, including the British Isles and Scandinavia, and North America.It is a deciduous shrub growing up to 4 m high...
(Berberis vulgaris
Berberis vulgaris
Berberis vulgaris /// is a shrub in the family Berberidaceae, native to central and southern Europe, northwest Africa and western Asia; it is also naturalised in northern Europe, including the British Isles and Scandinavia, and North America.It is a deciduous shrub growing up to 4 m high...
) or other susceptible Berberis
Berberis
Berberis , the barberries or pepperidge bushes, is a genus of about 450-500 species of deciduous and evergreen shrubs from 1-5 m tall with thorny shoots, native to the temperate and subtropical regions of Europe, Asia, Africa, North America and South America. They are closely related to the genus...
, Mahonia
Mahonia
Mahonia is a genus of about 70 species of evergreen shrubs in the family Berberidaceae, native to eastern Asia, the Himalaya, North America and Central America. They are closely related to the genus Berberis. Botanists disagree on the acceptability of the genus name Mahonia...
, or Mahoberberis species or cultivars. On barberry
Berberis
Berberis , the barberries or pepperidge bushes, is a genus of about 450-500 species of deciduous and evergreen shrubs from 1-5 m tall with thorny shoots, native to the temperate and subtropical regions of Europe, Asia, Africa, North America and South America. They are closely related to the genus...
, the basidiospore
Basidium
thumb|right|500px|Schematic showing a basidiomycete mushroom, gill structure, and spore-bearing basidia on the gill margins.A basidium is a microscopic, spore-producing structure found on the hymenophore of fruiting bodies of basidiomycete fungi. The presence of basidia is one of the main...
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
Berberis
Berberis , the barberries or pepperidge bushes, is a genus of about 450-500 species of deciduous and evergreen shrubs from 1-5 m tall with thorny shoots, native to the temperate and subtropical regions of Europe, Asia, Africa, North America and South America. They are closely related to the genus...
leaves by basidiospores, are the sexual stage of the fungus
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
life cycle
Biological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
. When a receptive hypha
Hypha
A hypha is a long, branching filamentous structure of a fungus, and also of unrelated Actinobacteria. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium; yeasts are unicellular fungi that do not grow as hyphae.-Structure:A hypha consists of one or...
from one pycnium has been fertilized by pycniospores (or spermatia) from a mating type compatible pycnium, its haploid cells become dikaryotic. The fertilized hypha
Hypha
A hypha is a long, branching filamentous structure of a fungus, and also of unrelated Actinobacteria. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium; yeasts are unicellular fungi that do not grow as hyphae.-Structure:A hypha consists of one or...
forms an aecium, on the underside of the barberry
Berberis
Berberis , the barberries or pepperidge bushes, is a genus of about 450-500 species of deciduous and evergreen shrubs from 1-5 m tall with thorny shoots, native to the temperate and subtropical regions of Europe, Asia, Africa, North America and South America. They are closely related to the genus...
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 cereal
Cereal
Cereals are grasses cultivated for the edible components of their grain , composed of the endosperm, germ, and bran...
s by penetrating through stomata. After infecting a cereal
Cereal
Cereals are grasses cultivated for the edible components of their grain , composed of the endosperm, germ, and bran...
plant, the aecispores develop and form uredia under the plants epidermis
Epidermis (botany)
The epidermis is a single-layered group of cells that covers plants' leaves, flowers, roots and stems. It forms a boundary between the plant and the external environment. The epidermis serves several functions, it protects against water loss, regulates gas exchange, secretes metabolic compounds,...
, these produce the dikaryotic urediniospores. These uredia eventually rupture the plant's epidermis and again spread by the wind to nearby cereal
Cereal
Cereals are grasses cultivated for the edible components of their grain , composed of the endosperm, germ, and bran...
plants, continuing the lifecycle.
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. 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.
Left: thin section through a Chara antheridium, note the layer of non-reproductive cells around the inner reproductive cells; right: spermatozoids in long strings of spermatozoid-producing cells from an antheridium.
Ug99
Ug99, which has the designation of TTKS, is a raceRace (biology)
In biology, races are distinct genetically divergent populations within the same species with relatively small morphological and genetic differences. The populations can be described as ecological races if they arise from adaptation to different local habitats or geographic races when they are...
of black stem rust (Puccinia graminis tritici).
It was discovered in Uganda, Africa in 1999 and has since spread to many other African countries. It is a race of black stem rust TTKS caused by the fungus Puccinia graminis tritici and is historically the most devastating fungal disease to wheat and barley that can result in 100% crop loss. This strain of wheat rust, located in Ugandan wheat fields, received its abbreviated name based on the country and year in which it was found. Seven races belonging to the Ug99 lineage are now known and have spread to various wheat-growing countries in the eastern African highlands, as well as Zimbabwe, South Africa, Sudan, Yemen, and Iran. While it is devastating South Africa and Middle Eastern wheat production and is expected to make its way into Pakistan and the Indian subcontinent, it has yet to enter the United States. This exclusive variety of wheat rust is devastating to the production of the crop because it kills 80-90% of global wheat cultivars under suitable conditions, according to the Food and Agricultural Organization of the United Nations. Worldwide, this strain of wheat stem rust is a threat to global wheat production threatening food securities in developing countries.
It is virulent
Virulence
Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...
to the great majority of wheat
Wheat
Wheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...
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
Uganda
Uganda , officially the Republic of Uganda, is a landlocked country in East Africa. Uganda is also known as the "Pearl of Africa". It is bordered on the east by Kenya, on the north by South Sudan, on the west by the Democratic Republic of the Congo, on the southwest by Rwanda, and on the south by...
in 1999 and has spread throughout the highlands of East Africa
East Africa
East Africa or Eastern Africa is the easterly region of the African continent, variably defined by geography or geopolitics. In the UN scheme of geographic regions, 19 territories constitute Eastern Africa:...
. In January 2007, spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s blew across to Yemen
Yemen
The Republic of Yemen , commonly known as Yemen , is a country located in the Middle East, occupying the southwestern to southern end of the Arabian Peninsula. It is bordered by Saudi Arabia to the north, the Red Sea to the west, and Oman to the east....
, and north into Sudan
Sudan
Sudan , officially the Republic of the Sudan , is a country in North Africa, sometimes considered part of the Middle East politically. It is bordered by Egypt to the north, the Red Sea to the northeast, Eritrea and Ethiopia to the east, South Sudan to the south, the Central African Republic to the...
. In March 2007, FAO
Fão
Fão is a town in Esposende Municipality in Portugal....
announced its concern regarding the spread through Iran
Iran
Iran , officially the Islamic Republic of Iran , is a country in Southern and Western Asia. The name "Iran" has been in use natively since the Sassanian era and came into use internationally in 1935, before which the country was known to the Western world as Persia...
based on Iranian authorities report.
Gene Resistance
Unlike other variations and species of wheat rust, Ug99 and its variants differ from other strains of the Black Stem Rust (BSR) pathogen due to their ability to overcome resistance genes in wheat that have been durable against the BSR pathogen for decades. These resistant Sr genes, of which 50 are known, confer different resistance to stem rust. The virulence in Uganda was associated with Sr31 and is specific to Ug99.The massive losses of wheat that have occurred have been devastating, but in recent years the wheat rust epidemic has been effectively controlled through selection and breeding for these Sr genes. The United States Department of Agriculture (USDA) researchers are testing genes to substantiate their Ug99 resistance, which will ultimately aid in developments of wheat varieties that will be able to fight off the rust. USDA researchers have stated that resistance has been identified in spring wheat land races and that they are now studying winter wheat land races where resistance is more probable. Due to the fact that the screening of the winter races is more challenges, results from the studies are not expected for another five to seven years. In addition to the research being conducted by the USDA, The United Kingdom’s Department for International Development (DFID), along with The Bill & Melinda Gates Foundation, announced in February 2011 that they will by granting $40 million dollars to a global project led by Cornell University to combat deadly strains of Ug99. The five-year grant to the Durable Rust Resistance in Wheat (DRRW) project will support attempts to identify new resistance genes as well as reproduce and distribute rust resistant wheat seeds to farmers.Life Cycle
Ug99 is a member of the Phylum Basidiomycota within the Kingdom Fungi. The characteristic rust color on stems and leaves is typical of a general stem rust as well as any variation of this type of fungus. Different from most fungi, the rust variations have five spore stages and alternate between two hosts. Wheat is the primary host and barberry is the alternate host.The first stage, referred to as Stage 0, begins in the Spermogonium and is the sexual reproduction stage. The Spermogonia emanates sweet nectar that attracts flies while encouraging the spermatia and receptive hyphae to “meet”. When the nectar is released and the flies come to drink the liquid the spermatia attach to the fly and are placed on the hyphae by way of the fly, thus beginning sexual reproduction. This step in the cycle is very important, and if it does not take place, the next phase will not occur. Stage 1 takes place in the aeciospores of the aecia. These spores are binucleate and are formed in a chainlike series. They are born on barberry and are spread by the wind and are the spores that actually infect the wheat. Urediniospores in the uredia are the summer spores of the following stage, Stage 2, and are the source of the characteristic rust spots that arise. Urediniospores are the most damaging spore to the wheat and can infect other plants throughout the spring and summer months due to the spores being distributed by the wind. Stage 3, referred to as the resting stage, gives rise to teliospores, also called resting spores, in the telia. This is an overwintering stage, and when spring comes around, each teliospore is capable of germination of basidia and basidiospres. The final stage, Stage 4, is the transitional stage where the reproductive basidiospores in the basidia infect the barberry host.
History of Stem Rust
The fungal ancestors of stem rust have infected grasses for millions of years and wheat crops for as long as they have been grown. According to Jim Peterson, professor of wheat breeding and genetics at Oregon State UniversityOregon State University
Oregon State University is a coeducational, public research university located in Corvallis, Oregon, United States. The university offers undergraduate, graduate and doctoral degrees and a multitude of research opportunities. There are more than 200 academic degree programs offered through the...
, "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.
While Ug99 wasn’t discovered until 1999, stem rust has been an ongoing problem dating back to Aristotle’s time (384-322 B.C). An early ancient practice by the Romans was one where they would sacrifice red animals such as foxes, dogs, and cows to Robigo or Robigus, the rust god. They would perform this ritual in the spring during a festival known as the Robigalia in hopes of the wheat crop being spared from the destruction caused by the rust. Weather records from that time have been observed and it has been speculated that the fall of the Roman Empire was due to a string of rainy seasons in which the rust would have been more harsh, resulting in reduced wheat harvests. The first laws banning barberry were established in 1660 in Rouen, France. This was due to the fact that European farmers had correlated a relationship between barberry and stem rust epidemics in wheat. The law banned the planting of barberry near wheat fields and was the first of its kind before the parasitic nature of stem rust was discovered in the 1700’s.
Two Italian scientists Fontana and Tozzetti first explained the stem rust fungus in wheat in 1767. Thirty years later it received its name, Puccinia graminis, by Persoon and in 1854 the Tulasne brothers discovered the characteristic five-spore stage that is known to some stem rust species. The brothers were also able to make a connection between the red (urediniospore) and black (teliospore) spores as different stages within the same organism, but the rest of the stages remained unknown.
Anton de Bary later conducted experiments to observe the beliefs of the European farmers regarding the relationship between the rust and barberry plants and after successful attempts to connect the basidiospores of the basidia stage to barberry, he also identified that the aeciospores in the aecia stage reinfect the wheat host. Upon de Bary’s discovery of all five spore stages and their need for barberry as a host, John Craigie, a Canadian pathologist, identified the function of the spermogonium in 1927.
Due to the useful nature of both barberry and wheat plants, they were eventually brought to the United States by European colonists. Barberry was used for many things like making wine and jams from the berries to tool handles from the wood. Ultimately, as they did in Europe, the colonists began to notice a relationship between barberry and stem rust epidemics in wheat. Laws were enacted in many New England colonies, but as the farmers moved west, the problem with the stem rust moved with them and began to spread to many areas, creating a devastating epidemic in 1916. It wasn’t until two years later in 1918 that the United States created a program to remove barberry. The program was one that was supported by state and federal entities and was prompted by the looming fear of food supplies during the war. The “war against barberries” was waged and called upon the help of citizens through radio and newspaper advertisements, pamphlets, and fair booths asking for help from all in the attempt to rid the barberry bushes of their existence. Later, in 1975-1980, the program was reestablishes back to state jurisdiction. Once this happened, a federal quarantine was established against the sale of stem rust susceptible barberry in those states that were part of the program. A barberry testing program was created to ensure that only the species of barberry and other variations of plants that are immune to stem rust will be grown in the quarantine area.