Wheat leaf rust
||This article's introduction may be too long for the overall article length. (December 2012)|
|Wheat leaf rust|
|Common names:||Brown rust
|Causal agents:||Puccinia triticina|
Puccinia dispersa f. sp. tritici Erikss. & E.Henn. 1894
Wheat leaf rust, is 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. Plant breeders have tried to improve yield quantities in crop 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 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 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 plant 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.
Wheat leaf rust is spreads via airborne spores. Five types of spores are formed in the life cycle. Uredospores, teleutospores, and basidiospores develop on wheat plants and pycnidiospores 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. In the Asian Subcontinent, the spores cannot survive the hot dry weather but are re-introduced every year from the Himalayas or surrounding hills, possibly coming from Berberis spp, Thalictrum flavum and Muehlenbergia huglet which is a main reason for bread mouldes or even some grasses. Wheat rust pathogens are biotrophic and require living plant cells to survive.
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. which it will overwinter on. 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.
The pathogen has an asexual and sexual cycle. In North America, South America and Australia the pathogen only undergoes its asexual cycle. However this does not seem to be a disadvantage to it, and wheat leaf rust has many races with different virulence. The sexual life cycle of wheat leaf rust requires a different host species, Thalictrumn spp.
Small brown pustules develop on the leaf blades in a random scatter distribution. They may group into patches in serious cases. Infectious spores are transmitted via the soil. 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.
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