Leaf Rust of Wheat

Leaf Rust of Wheat

Leaf rust has not enjoyed the notoriety of stem rust and yet it has a much greater impact on modern wheat production. It is the most widespread and in many areas the most common disease of this crop. Leaf rust has also been called brown rust, orange rust and dwarf rust. It can be found in many wheat growing areas including the Great Plains on a yearly basis. In Nebraska alone, monetary losses associated with the disease are estimated at $5.5 million per year. The causal organism of the disease is Puccinia recondita Roberge ex Desmaz. f. sp. tritici (Eriks. & E. Henn.) D.M. Henderson. Several races of the fungus have been identified. Wheat in any combination of characteristics , winter and spring seeded, hard and soft, red and white, is the primary host. The fungus is weakly virulent on some cultivars of barley and certain grass species from the Aegilops and Agropyron genera. Hosts other than wheat do not play an important role in the development of epidemics. Puccinia recondita f. sp. tritici is macrocyclic. However, the aecial and pycnial stages are rare in North America because alternate hosts, primarily meadow rue and other Thalictrum spp., are not common in this area. This is also the case in South America and Asia. The absence of an alternate host also diminishes the role of teliospores in the life cycle of the fungus. Puccinia recondita f. sp. tritici overwinters as mycelium and urediospores on green plant tissue. Winter survival of the fungus is most common in northern Mexico and Texas. In the Great Plains it may survive the winter on fall seeded wheat as far north as central Kansas. The pathogen may also survive farther north if the wheat is snow covered during the winter. Researchers have also reported that winter survival increases when wheat is seeded directly into stubble of the previous crop (no-till). In areas of winter wheat production, the fungus may survive between crops on volunteer wheat. The more southern regions generally serve as the starting point for yearly epiphytotics. Urediospores produced in these areas are wind blown to northern growing areas and serve as primary inoculum. These spores can be transported hundreds of miles on wind currents and still maintain their viability. Urediospores are deposited on leaf surfaces and germinate if favorable environmental conditions exist. Temperature and leaf wetness are the factors which limit development of leaf rust. The daytime temperature range for disease development is 60^o F to 80^oF, with an optimum around 70^oF. The minimum leaf wetness period required for spore germination at the optimum temperature is four hours. If the temperature falls below the optimum, the leaf wetness period required for germination increases. The most beneficial type of leaf moisture is dew since very wet conditions may retard spore dispersal. Once urediospores have germinated they form and appressorium over stomata. The host is penetrated by an infection peg and a haustorium forms within the host cell. Mycelium and more urediospores are produced in a structure called a uredium (pustule). These spores serve as secondary inoculum. Secondary or repeating cycles may be completed in 7 to 10 days allowing for many generations of urediospores per season. As the infected plant matures, teliospores are produced instead of urediospores and the fruiting structure is referred to as a telium. Fall infection typically occurs when the pathogen has survived the summer on volunteer wheat. Urediospores are wind blown from the volunteer to fall seeded wheat. Fall infection is not usually detrimental to the host.

Symptoms

The most common sites for symptoms to appear is on leaf blades and sheaths, however glumes and awns may occasionally become infected and exhibit symptoms. Uredia are seen as small orange blisters or pustules on infected tissue. The epidermis covering the pustule ruptures and the structure takes on a dusty appearance. Orange spores are easily dislodged and may cover clothing, hands or implements. When the infection is severe leaves dry out and die. Since inoculum is blown into a given area, symptoms are often seen on upper leaves first. As plants mature, the orange urediospores are replaced by black teliospores. Pustules containing these spores are black and shiny since the epidermis does not rupture. Yield loss often occurs as a result of infection by Puccinia recondita f. sp. tritici. Heavy infection which extends to the flag leaf results in a shorter period of grain fill and small kernels.

Plant Health Management

Resistant Cultivars.

Fungicide Application.

Compounds labeled for leaf rust control include: benomyl (Benlate), mancozeb ( Dithane, Manzate 200, Penncozeb), propiconazole (Tilt) and triadimefon (Bayleton). The seed treatment difenocanazole (Dividend) is also labeled for fall season control. (1997). Fungicide application may or may not be economically feasible in a given year. State and national survey pathologists monitor the progression of the wheat rusts each year so it is important to have the information they provide available when making a decision on whether or not to treat the crop.

References

Cook, R.J. and Veseth, R.J. 1991. Wheat health management. APS Press, St. Paul, MN. 152 pp.

Dickson, J. G. 1956. Diseases of field crops, 2nd ed. McGraw-Hill Book company, Inc. New York, NY. 517 pp.

Watkins, J.E. 1995. Rust diseases of wheat: leaf, stem, and stripe. Cooperative Extension, Institure of Agriculture and Natural Resources, University of Nebraska - Lincoln. NebGuide G95-1245-A.

Wiese, M.V. ed. 1987. Compendium of wheat diseases, 2nd ed. APS Press, St. Paul, MN. 112 pp.

Willis, W.G. 1984. Wheat diseases. Cooperative Extension Service, Kansas State University, Manhattan, KS. Publication S-23, 31 pp.

Useful Links

Texas A&M University

Caveat

This description is presented for information only and no endorsement is intended for products listed, nor criticism meant for products not mentioned. Always consult the product label before purchasing and using any pesticide.

Material contained on the Links from the page are the responsibility of the linked page's author(s).

This page was researched and drafted by: Jane Christensen, Department of Plant Pathology, University of Nebraska-Lincoln

Disease images were provided by: Dr. David Wysong, Department of Plant Pathology, University of Nebraska-Lincoln

Questions, Comments, Complaints and Complements?

This page is authored and maintained by:
Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

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