Wheat leaf rust

Wheat leaf rust (''Puccinia triticina'') is a fungal disease that affects wheat, barley, rye stems, leaves and grains. In temperate zones it is destructive on winter wheat because the pathogen overwinters. 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 epidemics in North America, Mexico and South America and is a devastating seasonal disease in India. ''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 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. 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, 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 graminicolous