Wheat leaf rust (''Puccinia triticina'') is a fungal disease that affects

wheat Wheat is a grass widely cultivated for its seed, a cereal grain that is a worldwide staple food. The many species of wheat together make up the genus ''Triticum'' ; the most widely grown is common wheat (''T. aestivum''). The archaeologi ...

barley Barley (''Hordeum vulgare''), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains, particularly in Eurasia as early as 10,000 years ago. Globally 70% of barley pr ...

rye Rye (''Secale cereale'') is a grass grown extensively as a grain, a cover crop and a forage crop. It is a member of the wheat tribe (Triticeae) and is closely related to both wheat (''Triticum'') and barley (genus ''Hordeum''). Rye grain is u ...

stems, leaves and grains. In

temperate zones In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...

it is destructive on

winter wheat Winter wheat (usually ''Triticum aestivum'') are strains of wheat that are planted in the autumn to germinate and develop into young plants that remain in the vegetative phase during the winter and resume growth in early spring. Classification ...

because the pathogen

overwinter Overwintering is the process by which some organisms pass through or wait out the winter season, or pass through that period of the year when "winter" conditions (cold or sub-zero temperatures, ice, snow, limited food supplies) make normal activi ...

s. Infections can lead up to 20% yield loss, which is exacerbated by dying leaves, which fertilize the fungus. The pathogen is a '' Puccinia'' rust fungus. It is the most prevalent of all the wheat rust diseases, occurring in most wheat-growing regions. It causes serious

epidemic An epidemic (from Ancient Greek, Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of patients among a given population within an area in a short period of time. Epidemics ...

s in

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...

and

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...

and is a devastating seasonal disease in

India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...

. ''P. triticina'' is

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 hosts are required for the parasite to complete its life cycle ...

, requiring two distinct hosts (alternate hosts).

Host resistance

Plant breeders have tried to improve yield quantities in crops like wheat from the earliest times. In recent years, breeding for resistance against disease proved to be as important for total wheat production as breeding for increase in yield. The use of a single resistance gene against various pests and diseases plays a major role in resistance breeding for cultivated crops. The earliest single resistance gene was identified as effective against yellow rust. Numerous single genes for leaf rust resistance have since been identified. Leaf rust resistance gene is an effective adult-plant resistance gene that increases resistance of plants against ''P. recondita'' f.sp. ''tritici'' (UVPrt2 or UVPrt13) infections, especially when combined with genes Lr13 and gene Lr34 (Kloppers & Pretorius, 1997). Lr37 originates from the French cultivar VPM1 (Dyck & Lukow, 1988). The line RL6081, developed in Canada for Lr37 resistance, showed seedling and adult-plant resistance to leaf, yellow and stem rust. Crosses between the French cultivars will therefore introduce this gene into local germplasm. Not only will the gene be introduced, but the genetic variation of South African cultivars will also increase. Molecular techniques have been used to estimate genetic distances among different wheat cultivars. With the genetic distances known predictions can be made for the best combinations concerning the two foreign genotypes carrying gene Lr37, VPMI and RL6081 and local South African cultivars. This is especially important in wheat with its low genetic variation. The gene will also be transferred with the least amount of backcrosses to cultivars genetically closest to each other, generation similar genetic offspring to the recurrent parent, but with gene Lr37, Genetic distances between near isogenic lines (NILs) for a particular gene will also give an indication of how many loci, amplified with molecular techniques, need to be compared in order to locate putative markers linked to the gene.

Nomenclatural history

The ''Puccinia'' species causing wheat leaf rust has been called by at least six different names since 1882, when G. Winter (1882) described the ''Puccinia rubigo-vera''. During this time, wheat leaf rust was interpreted as a specialized form of ''P. rubigo-vera''. Later, Eriksson and Henning (1894) classified the fungi as ''P. dispersa'' f.sp. ''tritici''. In 1899, Eriksson concluded that the rust should be considered a separate authentic species. Due to this, he described the fungi as ''P. triticina''. This name was used by Gauemann (1959) in his comprehensive book about rust fungi of middle Europe. Mains (1933) was among the first scientists to use a species name with a broad species concept for wheat leaf rust. He considered ''P. Rubigo-vera'' a current name and posited that 32 binomials were synonyms of that species. Cummins and Caldwell (1956) CABDbr>19561101781
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built off the broad species concept and also discussed the validity of ''P. rubigo-vera'', which was based on a uredinial-stage

basionym In the scientific name of organisms, basionym or basyonym means the original name on which a new name is based; the author citation of the new name should include the authors of the basionym in parentheses. The term "basionym" is used in both botan ...

. Finally, they introduced ''P. recondita'' as the oldest valid name for grass-based rusts, including wheat leaf rust. Their idea and publication was followed by Wilson & Henderson (1966) in another comprehensive rust flora (viz. British Rust Flora). Wilson and Henderson (1966) also used a broad species concept for ''P. recondita'' and divided this broad species into 11 different formae speciales. The accepted name for wheat leaf rust in their flora was ''P. recondita'' f.sp. ''tritici''. Cummins (1971), in his rust monograph for

Poaceae Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species cultivated in lawns an ...

, introduced an ultra-broad species concept for ''P. recondita'' and listed 52 binomials as its synonyms. This concept gained attention among mycologists and plant pathologists around the world, hence why ''P. recondita'' is still found as an appropriate name for wheat leaf rust in some publications. A stream of thought opposite to broad morphologically-based concepts also gained traction among uredinologists. This idea was introduced into the classification of

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