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

wheat Wheat is a group of wild and crop domestication, domesticated Poaceae, grasses of the genus ''Triticum'' (). They are Agriculture, cultivated for their cereal grains, which are staple foods around the world. Well-known Taxonomy of wheat, whe ...

barley Barley (), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains; it was domesticated in the Fertile Crescent around 9000 BC, giving it nonshattering spikele ...

rye Rye (''Secale cereale'') is a grass grown extensively as a grain, a cover crop and a forage crop. It is grown principally in an area from Eastern and Northern Europe into Russia. It is much more tolerant of cold weather and poor soil than o ...

stems, leaves and grains. In temperate zones it is destructive on

winter wheat Winter wheat (usually ''Common wheat, 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. C ...

because the pathogen overwinters. Infections can lead up to 20% yield loss. The pathogen is a '' Puccinia''

rust fungus Rusts are fungal plant pathogens of the order Pucciniales (previously known as Uredinales) causing plant fungal diseases. An estimated 168 rust genera and approximately 7,000 species, more than half of which belong to the genus ''Puccinia'', are ...

. It is the most prevalent of all the wheat rust diseases, occurring in most wheat-growing regions. It causes serious

epidemic An epidemic (from Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of hosts in a given population within a short period of time. For example, in meningococcal infection ...

s in

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and

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and is a devastating seasonal disease in

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. ''P. triticina'' is heteroecious, requiring two distinct hosts (alternate hosts).

Host resistance

Plant breeders have tried to improve yield quantities in crops like wheat from the beginning of agriculture. From the 20th century, 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, raising the genetic variation of South African cultivars. 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 be transferred with the least amount of backcrossing to cultivars genetically closest to each other, generation similar genetic offspring to the recurrent parent, but with gene Lr37 present. Genetic distances between near isogenic lines (NILs) for a particular gene will give an indication of how many loci, amplified with molecular techniques, need to be compared 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. George Baker Cummins and Ralph Merrill Caldwell (1956) 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 ( ), also called Gramineae ( ), is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos, the grasses of natural grassland and species cultivate ...

, introduced an ultra-broad species concept for ''P. recondita'' and listed 52 binomials as its synonyms. 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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