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Stem rust

stem rust of wheat, stem rust
Dicaeoma anthistiriae
Puccinia albigensis
Puccinia anthistiriae
Puccinia brizae-maximae
Puccinia cerealis
Puccinia elymina
Puccinia favargeri
Puccinia graminis f macrospora
Puccinia graminis fsp avenae
Puccinia graminis fsp secalis
Puccinia graminis fsp 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 and cereal rusts are caused by the fungus Puccinia graminis and are a significant disease affecting cereal crops Crop species which are affected by the disease include bread wheat, durum wheat, barley and triticale1 These diseases have affected cereal farming throughout history Since the 1950s, wheat strains bred to be resistant to stem rust have become available2 Fungicides effective against stem rust are available as well3

In 1999 a new virulent race of stem rust was identified that most current wheat strains show no resistance against The race was named TTKSK eg isolate Ug99, named after the country where it was identified Uganda and the year of its discovery 19994 It spread to Kenya, then Ethiopia, Sudan and Yemen, and is becoming more virulent as it spreads4 An epidemic of stem rust on wheat caused by race TTKSK is currently spreading across Africa, Asia and the Middle East and is causing major concern due to the large numbers of people dependent on wheat for sustenance 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 identified4

An outbreak of another virulent race of stem rust, TTTTF, took place in Sicily in 2016, suggesting that the disease is now returning to Europe2

Contents

  • 1 Biology
  • 2 Pathology
  • 3 Signs and symptoms
    • 31 On wheat
    • 32 On barberry
  • 4 Life cycle
    • 41 Life cycle on barberry
    • 42 Life cycle without barberry
  • 5 Spore dispersal
  • 6 Wheat stem rust resistance genes
  • 7 History of stem rust
  • 8 See also
  • 9 References
  • 10 External links

Biologyedit

Model of a spore of puccinia graminis, late 19th century, Botanical Museum Greifswald

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 hordei, barley
  • Puccinia graminis f sp lolii
  • Puccinia graminis f sp poae
  • Puccinia graminis f sp secalis, rye, barley
  • Puccinia graminis f sp tritici, wheat, barley

P graminis 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

There are multiple pathotypes including QCC and MCC affecting barley, within forma specialis tritici5

Pathologyedit

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 stalk4 Infected plants produce fewer tillers and set fewer seed, and in cases of severe infection the plant may die Infection can reduce what is an apparently healthy crop about three weeks before harvest into a black tangle of broken stems and shriveled grains by harvest1

Stem rust of cereals causes yield losses in several ways:6

  • Fungus absorbs nutrients that would otherwise be used for grain development6
  • Pustules break through epidermis, which disrupt the plant's control of transpiration and can lead to desiccation and infection by other fungi6
  • Interference with plant vascular tissue leads to shriveled grains6
  • The fungus weakens the stems, which can lead to lodging falling over In severe cases lodging can make mechanical harvesting impossible6

Signs and symptomsedit

On wheatedit

Stem rust on wheat is characterized by the presence of uredinia on the plant, which are brick-red, elongated, blister-like pustules which are easily shaken off1 They most frequently occur on the leaf sheaths, but are also found on stems, leaves, glumes and awns1 On leaves they develop mostly on the underside but may penetrate to the upperside1 On leaf sheaths and glumes pustules rupture the epidermis, giving a ragged appearance1

Towards the end of the growing season black telia are produced1 For this reason stem rust is also known as 'black rust'1 The telia are firmly attached to the plant tissue1

The site of infection is a visible symptom of the disease

On barberryedit

Pycnia appear on barberry plants in the spring, usually in the upper leaf surfaces6 They are often in small clusters and exude pycniospores in a sticky honeydew6 Five to ten days later, cup-shaped structures filled with orange-yellow, powdery aeciospores break through the lower leaf surface6 The aecial cups are yellow and sometimes elongate to extend up to 5 mm from the leaf surface6

Life cycleedit

Life cycle of Puccinia graminis

Like other Puccinia species, P graminis is an obligate biotroph it colonizes living plant cells and has a complex life cycle featuring alternation of generations The fungus is heteroecious, requiring two hosts to complete its life cycle - the cereal host and the alternate host6 There are many species in Berberis and Mahonia that are susceptible to stem rust, but the common barberry is considered to be the most important alternate host1 P graminis is macrocyclic6 exhibits all five of the spore types that are known for rust fungi7

P graminis can complete its life cycle either with or without barberry the alternate host6

Life cycle on barberryedit

Due to its cyclical nature, there is no true 'start point' for this process Here, the production of urediniospores is arbitrarily chosen as a start point

Urediniospores are formed in structures called uredinia, which are produced by fungal mycelia on the cereal host 1–2 weeks after infection6 The urediniospores are dikaryotic contain two un-fused, haploid nuclei in one cell and are formed on individual stalks within the uredinium6 They are spiny and brick-red6 Urediniospores are the only type of spores in the rust fungus life cycle which are capable of infecting the host on which they are produced, and this is therefore referred to as the 'repeating stage' of the life cycle6 It is the spread of urediniospores which allows infection to spread from one cereal plant to another6 This phase can rapidly spread the infection over a wide area

Towards the end of the cereal host's growing season, the mycelia produce structures called telia6 Telia produce a type of spore called teliospores6 These black, thick-walled spores are dikaryotic6 They are the only form in which Puccinia graminis is able to overwinter independently of a host6

Each teliospore undergoes karyogamy fusion of nuclei and meiosis to form four haploid spores called basidiospores6 This is an important source of genetic recombination in the life cycle6 Basidiospores are thin-walled and colourless6 They cannot infect the cereal host, but can infect the alternative host usually barberry6 They are usually carried to the alternative host by wind

Once basidiospores arrive on a leaf of the alternative host, they germinate to produce a haploid mycelium which directly penetrates the epidermis and colonises the leaf6 Once inside the leaf the mycelium produces specialised infection structures called pycnia6 The pycnia produce two types of haploid gametes, the pycniospores and the receptive hyphae6 The pycniospores are produced in a sticky honeydew which attracts insects6 The insects carry pycniospores from one leaf to another6 Splashing raindrops can also spread pycniospores6 A pycniospore can fertilise a receptive hypha of the opposite mating type, leading to the production of a dikaryotic mycelium6 This is the sexual stage of the life cycle and cross-fertilisation provides an important source of genetic recombination6

This dikaryotic mycelium then forms structures called aecia, which produce a type of dikaryotic spores called aeciospores6 These have a worty appearance and are formed in chains - unlike the urediniospores which are spiny and are produced on individual stalks6 The chains of aeciospores are surrounded by a bell-like enclosure of fungal cells The aeciospores are able to germinate on the cereal host but not on the alternative host they are produced on the alternative host, which is usually barberry6 They are carried by wind to the cereal host where they germinate and the germ tubes penetrate into the plant6 The fungus grows inside the plant as a dikaryotic mycelium6 Within 1–2 weeks the mycelium produces uredinia and the cycle is complete6

Life cycle without barberryedit

Since the urediniospores are produced on the cereal host and can infect the cereal host, it is possible for the infection to pass from one year's crop to the next without infecting the alternate host barberry6 For example, infected volunteer wheat plants can serve as a bridge from one growing season to another6 In other cases the fungus passes between winter wheat and spring wheat, meaning that it has a cereal host all year round6 Since the urediniospores are wind dispersed, this can occur over large distances6 Note that this cycle consists simply of vegetative propagation - urediniospores infect one wheat plant, leading to the production of more urediniospores which then infect other wheat plants

Spore dispersaledit

Puccinia graminis produces all five of the spore types that are known for rust fungi6

Spores are typically deposited close to the source, but long-distance dispersal is also well documented1 The following three categories of long-distance dispersal are known to occur:1

  • Extremely long-distance dispersal

This can occur unassisted the robust nature of the spores allows them to be carried long distances in the air and then deposited by rain-scrubbing or assisted typically on human clothing or infected plant material that is transported between regions1 This type of dispersal is rare and is very difficult to predict1

  • Step-wise range expansion

This is probably the most common mode of long-distance dispersal and usually occurs within a country or region1

  • Extinction and recolonisation

This occurs in areas that have unsuitable conditions for year-round survival of Puccinia graminis - typically temperate regions where hosts are absent during either the winter or summer1 Spores overwinter or oversummer in another region and then recolonise when conditions are favorable1

Wheat stem rust resistance genesedit

A number of stem rust resistance genes Sr genes have been identified in wheat8 Some of them arose in bread wheat eg Sr5 and Sr6, while others have been bred in from other wheat species eg Sr21 from T monococcum or from other members of the tribe Triticeae eg Sr31 from rye and Sr44 from Thinopyrum intermedium

None of the Sr genes provide resistance to all races of stem rust For instance many of them are ineffective against the Ug99 lineage8 Notably Ug99 has virulence against Sr31, which was effective against all previous stem rust races Recently, a new stem rust resistance gene Sr59 from Secale cereale was introgressed into wheat, which provides an additional asset for wheat improvement to mitigate yield losses caused by stem rust Rahmatov et al, 2016

Singh et al, 2011 provide a list of known Sr genes and their effectiveness against Ug998

History of stem rustedit

The fungal ancestors of stem rust have infected grasses for millions of years and wheat crops for as long as they have been grown4 According to Jim Peterson, professor of wheat breeding and genetics at Oregon State University, "Stem rust destroyed more than 20% of US wheat crops several times between 1917 and 1935, and losses reached 9% twice in the 1950s," with the last US outbreak in 1962 destroying 52% of the crop4

While Ug99 wasn't discovered until 1999, stem rust has been an ongoing problem dating back to Aristotle's time 384-322 BC6 An early ancient practice by the Romans was one where they would sacrifice red animals such as foxes, dogs, and cows to Robigus fem Robigo, 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 rust6 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 harvests6 Laws banning barberry were established in 1660 in Rouen, France This was due to the fact that European farmers noticed a correlation between barberry and stem rust epidemics in wheat6 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 1700s6

Two Italian scientists, Fontana and Tozzetti, first explained the stem rust fungus in wheat in 17676 Thirty years later it received its name, Puccinia graminis, by Persoon, and in 1854 brothers Louis René and Charles Tulasne discovered the characteristic five-spore stage that is known to some stem rust species6 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 unknown6

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 host6 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 19276

Due to the useful nature of both barberry and wheat plants, they were eventually brought to North America by European colonists6 Barberry was used for many things like making wine and jams from the berries to tool handles from the wood6 Ultimately, as they did in Europe, the colonists began to notice a relationship between barberry and stem rust epidemics in wheat6 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 19166 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 war6 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 existence6 Later, in 1975-1980, the program was reestablished under state jurisdiction6 Once this happened, a federal quarantine was established against the sale of stem rust susceptible barberry in those states that were part of the program6 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 area6

See alsoedit

  • Chilean wheat cycle
  • List of Puccinia species

Referencesedit

  1. ^ a b c d e f g h i j k l m n o p q Singh, Ravi P; Hodson, David; Huerta-Espino, Julio; Jin, Yue; Njau, Peter; Wanyera, Ruth; Herrera-Foessel, Sybil and Ward, Richard W 2008 "Will Stem Rust Destroy The World's Wheat Crop" Advances in Agronomy 98: 272–309 doi:101016/S0065-21130800205-8 CS1 maint: Multiple names: authors list link
  2. ^ a b Bhattacharya, Shaoni 2017-02-09 "Deadly new wheat disease threatens Europe’s crops" Nature 542 7640: 145–146 doi:101038/nature201721424 
  3. ^ Wanyera, R; Macharia, J K; Kilonzo, S M; Kamundia, J W 2009-08-06 "Foliar Fungicides to Control Wheat Stem Rust, Race TTKS Ug99, in Kenya" Plant Disease 93 9: 929–932 ISSN 0191-2917 doi:101094/PDIS-93-9-0929 
  4. ^ a b c d e f Karen Kaplan A red alert for wheat July 22, 2009 BrandX/ LA Times
  5. ^ Jin, Y; Steffenson, Bj; Miller, Jd, 1994: "Inheritance of resistance to pathotypes QCC and MCC of Puccinia graminis fsp tritici in barley line Q21861 and temperature effects on the expression of resistance" Phytopathology 845: 452-455
  6. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af ag ah ai aj ak al am an ao ap aq ar as at au av aw ax ay az ba bb bc bd be bf bg bh bi bj bk Schumann, GL; Leonard, K J 2011 2000 "Stem rust of wheat black rust" The Plant Health Instructor doi:101094/PHI-I-2000-0721-01  Missing or empty |url= help; |access-date= requires |url= help
  7. ^ Peterson, Ronald H 1974 "The rust fungus life cycle" The Botanical Review 40: 453–513 doi:101007/BF02860021 
  8. ^ a b c Singh, Ravi P; Hodson, David P; Huerta-Espino, Julio; Jin, Yue; Bhavani, Sridhar; Njau, Peter; Herrera-Foessel, Sybil; Singh, Pawan K; Singh, Sukhwinder; Govindan, Velu 8 September 2011 "The Emergence of Ug99 Races of the Stem Rust Fungus is a Threat to World Wheat Production" Annual Review of Phytopathology 49 1: 465–481 doi:101146/annurev-phyto-072910-095423 

External linksedit

  • Borlaug Global Rust Initiative
  • FAO
  • Animation of stem rust life cycle

stem rust, stem rust barberry, stem rust disease, stem rust it scale, stem rust life cycle, stem rust of wheat, stem rust resistance, stem rust seeds, stem rust treatment, stem rust ug99


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