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wheat leaf rust
Wheat leaf rust is a fungal disease that affects wheat, barley and 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 exacerbated by dying leaves which fertilize the fungus. The pathogen is Puccinia rust fungus. ''Puccinia triticina'' causes 'black rust', ''P.recondita'' causes 'brown rust' and ''P.striiformis'' causes 'Yellow rust'. 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. All three types of ''Puccinia'' are heteroecious requiring two distinct and distantly related hosts (alternate hosts). Rust and the similar smut are members of the class Pucciniomycetes but rust is not normally a black powdery mass.
==Host resistance==
Plant breeders have tried to improve yield quantities in crops like wheat from the earliest times. In recent years, breeding for the 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, the 47th genes prevent crop losses due to ''Puccinia recondite'' Rob. Ex Desm. f.sp. ''tritici'' infections, which can range from 5–15% depending on the stage of crop development.
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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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