SDSU Small Grains and Feedstock Crops Pathology

Summary

Stem rust, caused by Puccinia graminis, has historically been one of the most economically important pathogens of the small grain cereals (wheat, barley, oats, and rye) in the U.S. Northern Great Plains (especially: Minnesota, North Dakota, and South Dakota). Annual yield losses due to stem rust in the region often exceeded 10% and were substantially greater during severe epidemics. This disease has been of minor importance in recent decades, especially for wheat; however a highly virulent race of the pathogen was identified in Africa that can infect the majority of the wheat and barley varieties currently grown in this region. This race, called Ug99 (or TTKST), has recently been found in the Middle-East (as of 2008) and could again become a serious issue for wheat and barley in the United States if it becomes established here. Please note that the author is in South Dakota (USA) and therefore some of this material may not be relevant to other locations or cropping systems.

Pathogen Information & Symptoms

Puccinia graminis is a heteroecious, macrocyclic rust. That is, it has five different spore stages and requires two different hosts to complete its life cycle. The most economically important stage of P. graminis is the uredinial stage. Spores of this stage (urediniospores) are asexual (clonal), produced during the growing season, and can re-infect the same host species that they were produced from. For example, urediniospors of P. graminis f.sp. avenae are produced from infected oat plants and can re-infect oats (the same plant, even). The other spore stages that are able to infect plants, i.e. aeciospores and basidiospores, do not infect the same host that they came from but a different host. For example, the aeciospores produced from barberry plants are only able to infect a grass host (e.g. wheat).

P. graminis has a relatively wide host range for a rust and is somewhat atypical in this manner for the genus Puccinia. Most members of this genus are only able to infect a limited number of host species, often a single genus for each of the uredinal and aecial stages. This species has several formae speciales (similar to sub-species) that tend to differ significantly in their host ranges, although there is some overlap with the f.sp. that are pathogenic on grasses. P. graminis can infect most of the foliar tissues of susceptible species and on the uredinal hosts (e.g. wheat), this includes the leaves, leaf sheaths, stems, and spikes (flowering tissues). On the alternate host (e.g. barberry), infection is usually limited to leaves. The table below contains the f.sp. that are pathogens of the small grain cereals and most pertinent to our research.

Several of the links below have more thorough descriptions of the lifecycle and biology of this pathogen.

Host Information

The most economically-important hosts of P. graminis are the small grain cereals as these are staple food crops throughout most of the temperate regions in the World (e.g. wheat). The other susceptible food crops (oat, barley, & rye) may be regionally important as food sources and all are used as livestock feed. Barley is also a main component in beer production. Losses to wheat and barley in the U.S. have been minimal for many decades, with only the occasional, minor epidemic in an individual state or region (e.g. 2% in the 1986 Kansas winter wheat crop - see USDA CDL Rust Loss Estimates). In fact, it can be a challenge to find wheat stem rust in a commercial wheat or barley field. In contrast, oat stem rust continues to regularly cause economic losses, although they also tend to be minor (<5%). As noted above, certain P. graminis f.sp. (or sub-species) also infect many other grass species and can be a problem locally (e.g. ryegrass seed production in the U.S. Pacific Northwest).

As noted above, most of the above-ground tissues on a susceptible plant can be infected by P. graminis; however, losses tend to be highest when there is a significant level of infection on the stems and flag leaves of the plant.

Environment

As with most rust pathogens, P. graminis requires free moisture on the leaves for the spores to germinate and infect. The optimal temperature for this pathogen are daytime temperatures near (85F) and night temperatures near (65F) with a prolonged dew period.

Urediniospores can be

Management

Control options for stem rust were limited at the beginning of the 20th century due to a highly variable pathogen population, the lack of stable resistance in the crops, and the low efficacy of the fungicides available at the time. In essence, the fact that the pathogen was unable to undergo sexual recombination

The USDA Barberry Eradication Program

Our Research in this Pathosystem.

We are re-investigating the epidemiology and integrated management of stem rust of wheat, barley, and oats. Specifically, we are researching infection models for use in a future stem rust risk advisory system for wheat and barley. We are also evaluating the efficacy of currently available fungicides and application technologies in the management of wheat stem rust under field conditions. This project is funded by a USDA-CSREES Critical Issues Grant.

Additional Images

Life cycle of Pgt (USDA-ARS-CDL)

Links

• USDA Cereal Disease Lab - Wheat Stem Rust Page / Rust Loss Estimates /
• American Phytopathological Society - Stem Rust Plant Disease Lesson (very complete)
• Borlaug Global Rust Initiate
• Wikipedia - Stem Rust Page
• Washing State University - Stem Rust Page
• University of Nebraska - Stem Rust Page
• USDA-APHIS - Stem Rust Regulatory Information (for Barberry)
• Broad Institute - Puccinia graminis Genome Database

References

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