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, which is exacerbated by dying leaves, which fertilize the fungus. The pathogen is Puccinia rust fungus. Puccinia graminis causes "stem or black rust", P. triticina causes "leaf or brown rust", and P. striiformis causes "strip or 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. 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% to 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 infections, especially when combined with genes Lr13 and gene Lr34. Lr37 originates from the French cultivar VPM1. 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 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
Fungal names are important. These are the keys to all information behind them. Then, an appropriate name can lead users to the right information. In the case of plant pathogenic fungi using an appropriate name is more important because of practical reasons. There are several examples among rust fungi of one species called with different names during different eras. However, one of the most interesting ones is the name for Puccinia species causing Wheat Leaf Rust. This species has been called by at least six different names since 1882, when G. Winter described the Puccinia rubigo-vera. For long time WLR interpreted as a specialized form of P. rubigo-vera. Later, Eriksoon and Henning put it under the P. dispersa f.sp. tritici. In 1899 and after some experiments Eriksson concluded that the rust should be considered as a separate authentic species. For this reason he described P. triticina. This name was used by Gaeumann in his comprehensive book about rust fungi of middle Europe. Mains was among the first scientists who used a species name with broad species concept for WLR. He considered P. rubigo-vera as current name and put 32 binomials as synonyms of that species. The next important article about naming WLR was published by Cummins and Caldwell. They considered the same broad species concept and also discussed the validity of P. rubigo-vera which was based on an uerdinial stage basionym. Finally, they introduced P. recondita as the oldest valid name for WLR and also other grasses. Their idea and publication was followed by Wilson & Henderson in another comprehensive rust flora viz. British Rust Flora. Wilson and Henderson also used a broad species concept for P. recondita and divided this broad species to 11 different formae speciales. The accepted name for WLR in their flora was P. recondita f.sp. tritici. Cummins in his rust monograph for Poaceae introduced an ultra-broad species concept for P. recondita and listed 52 binomials as its synonyms. Such a concept found great attention among mycologists and plant pathologists around the world and that is the reason we still can seeP. recondita as an appropriate name for WLR in some publications. There was another stream opposite to broad morphologically-based concept among uredinologists. In the case of graminicolous rust fungi this stream was started by Urban who introduced P. perplexans var. triticina as an appropriate name for WLR. To Urban’s understanding, a taxonomic name should reflect both morphology and ecology of the species. Savile was also among the uredinologists believing in narrowing the species concept and considered P. triticina as an authentic taxonomic name for WLR. Urban’s research continued and he put many morphological, ecological and also field experiences together. Finally he considered WLR as a part of Puccinia persistens species with aecial stage on Ranunculaceae members, totally different from P. recondita which produces its aecial stage on Boraginacec family members. His final name for this rust was P. persistens subsp. triticina. Recent molecular and also morphological studies proved Urban’s taxonomy for WLR. It seems after more than a century and after introducing several names, we have an appropriate name for WLR
Life cycle
Life cycle: Wheat leaf rust spreads via airborne spores. Five types of spores are formed in the life cycle: Urediniospores, teliospores, and basidiospores develop on wheat plants and pycniospores and aeciospores develop on the alternate hosts. The germination process requires moisture, and works best at 100% humidity. Optimum temperature for germination is between 15–20 °C. Before sporulation, wheat plants appear completely asymptomatic.
P. triticina has an asexual and sexual life cycle. In order to complete its sexual life cycle P. triticina requires a second host Thalictrum spp. on which it will overwinter. In places where Thalictrum does not grow, such as Australia, the pathogen will only undergo its asexual life cycle and will overwinter as mycelium or uredinia. The germination process requires moisture and temperatures between 15–20 °C. After around 10–14 days of infection, the fungi will begin to sporulate and the symptoms will become visible on the wheat leaves.
Epidemiology
Location is an important characteristic in the spread of wheat rust. Some places wheat rust can easily flourish and spread. In other areas, the environment is marginally suited for the disease. Urediniospores of the wheat rusts initiate germination within one to three hours of contact with free moisture over a range of temperatures depending on the rust. Urediniospores are produced in large numbers and can be blown considerable distances by the wind, but most urediniospores are deposited close to their source under the influence of gravity. Urediniospores are relatively resilient and can survive in the field away from host plants for periods of several weeks. They can withstand freezing if their moisture content is lowered to 20 to 30 percent. Viability rapidly decreases at moisture contents of more than 50 percent. Long-distance spread of urediniospores is influenced by wind patterns and the orientation of the spore to latitude. In general, spores move west to east due to the winds resulting from the rotation of the earth. At progressively higher latitudes, winds tend to take a more southerly component in the Northern Hemisphere and a northerly component in the Southern Hemisphere. Puccinia triticina can survive the same environmental conditions as the wheat leaf, provided infection but no sporulation has occurred. The fungus can infect in less than three hours in the presence of moisture and temperatures below 20 °C; however, more infections occur with longer exposure to moisture.
Symptoms
Small brown pustules develop on the leaf blades in a random scatter distribution. They may group into patches in serious cases. Onset of the disease is slow but accelerated in temperatures above 15 °C, making it a disease of the mature cereal plant in summer, usually too late to cause significant damage in temperate areas. Losses of between 5 and 20% are normal but may reach 50% in severe cases. Symptoms can range in severity from barely aesthetic to completely overrun on the leaf surface. On barberry leaf the disease appears as powdery yellow spots with aecia being dispersed from the underside of the leaf.
Control
Varietal resistance is important. Chemical control with triazole fungicides may be useful for control of infections up to ear emergence but is difficult to justify economically in attacks after this stage. Control often is not as common as prevention through the creation of genetically- resilient varieties and the removal of common barberry. Cultivars are the best method of controlling the disease and have been utilized for over 100 years. However, resistance linked to single genes have been out maneuvered by the pathogen adapting to new cultures. This is why the destruction of the alternate hosts are key to control. Early-maturing cultivars as well as spring wheat should be sown as early as possible to avoid peak rust periods. Self-sown wheat should be destroyed as not to further spread urediniospores at the end of harvest.
