Wheat leaf rust

Wheat leaf rust
Symptoms of wheat leaf rust
Common names	Brown rust Leaf rust
Causal agents	Puccinia triticina
Hosts	wheat
EPPO code	PUCCRT
Distribution	Worldwide

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.

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 (WLR). This species has been called by at least six different names since 1882, when G. Winter (1882) described the Puccinia rubigo-vera.^[1] For long time WLR interpreted as a specialized form of P. rubigo-vera. Later, Eriksoon and Henning (1894) 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 (1959)^[2] in his comprehensive book about rust fungi of middle Europe. Mains (1933) was among the first scientists who used a species name with broad species concept for WLR.^[3] 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 (1956). 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 (1966) in another comprehensive rust flora viz. British Rust Flora. Wilson and Henderson (1966)^[4] 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 (1971) in his rust monograph for Poaceae introduced an ultra-broad species concept for P. recondita and listed 52 binomials as its synonyms.^[5] Such a concept found great attention among mycologists and plant pathologists around the world and that is the reason we still can see P. 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 (1969) who introduced P. perplexans var. triticina as an appropriate name for WLR.^[6] To Urban’s understanding, a taxonomic name should reflect both morphology and ecology of the species. Savile (1984) was also among the uredinologists believing in narrowing the species concept and considered P. triticina as an authentic taxonomic name for WLR.^[7] 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.^[8] Interestingly, recent molecular and also morphological studies proved Urban’s taxonomy for WLR.^[9] It seems after more than a century and after introducing several names, we have an appropriate name for WLR

Life cycle

Wheat leaf rust 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.^[10] 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. 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.^[11]

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.

Symptoms

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.

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

See also

• List of Puccinia species
• Wheat diseases
• Rust (fungus)
• Rice blast
• Stem rust

Notes

References

• Singh, Prof. V.; Dr. P. C. Pandey; Dr. D. K. Jain (2008). A Text Book of Botany. India: Rastogi. p. 15.132. ISBN 978-81-7133-904-4. 
• US Department of Agriculture