Stem Rust of the Small Grain Cereals (wheat, barley, oats, & rye)

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 United States. The states with the highest losses have historically been those in the Northern Great Plains, particularly North Dakota, Minnesota, and South Dakota (where the author is located). In fact, losses in this region regularly exceeded 10% and were much higher 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.

Our Research in this Pathosystem

We are studying the epidemiology and integrated management of stem rust of wheat, barley, and oats. Specifically, we are evaluating infection models for use in a future stem rust risk advisory system for wheat and barley. We are also determining the efficacy of currently available fungicides and application technologies in the management of stem rust under field conditions. This project is funded by a USDA-CSREES Critical Issues Grant. The PI of the program, Dr. Jeff Stein, is also coordinating an effort to investigate fungicide efficacy in multiple locations and wheat classes through uniform fungicide trials. To see preliminary results from a 2008 field trial in Brookings, SD click here.

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) and not barberry itself.

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 fungal genus are only able to infect a limited number of host species, often a single genus for each of the uredinal and aecial stages. In comparison, P. graminis has several formae speciale (abbrev: f.sp. / similar to sub-species) that can differ significantly in their host ranges, although there is some overlap with the f.sp. that are pathogenic on grasses. The table below contains the f.sp. that are pathogens of the small grain cereals and most pertinent to our research efforts.

Species
Primary Hosts in U.S.
(Uredinial stage)
Puccinia graminis f.sp. avenae
oat (Avena sativa),
wild oat (A. fatua)
Occasionally causes economic losses in commercial oats crops.
Puccinia graminis f.sp. secalis
rye (Secale cereale),
barley (Hordeum vulgare)
Of limited economic importance in recent years.
Puccinia graminis f.sp. tritici
wheat (Triticum aestivum), Durum ( T. turgidum), barley (Hordeum vulgare), Quack grass (Elytrigia repens), Foxtail Barley (Hordeum jubatum)
Occasionally causes economic losses in commercial wheat or barley crops. Most have been minor <5% in recent years.

P. graminis can infect most of the foliar tissues of susceptible species. For the uredinial 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 and petioles. The urediniospores produced from the uredinia (pustules) tend to be reddish-brown and are darker than those of the leaf and stripe rust pathogens. These pustules usually turn dark-brown to black by the end of the growing season as they switch to producing teliospores. As with other rust pathogens, P. graminis requires free moisture on the leaves for the spores to germinate and infect. The optimal conditions for this pathogen are daytime temperatures near 85F and night temperatures near 65F with a prolonged dew period at night. 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 other grass species and can be a problem locally (e.g. ryegrass seed production in the U.S. Pacific Northwest).

Management

Control options for stem rust were during the first half of the 20th century due to a highly variable pathogen population, the lack of stable resistance in the host crops, and low efficacy of the fungicides available at the time. In order to combat stem rust, the USDA initiated the Barberry Eradication Program in 1918. This program ran until 1975 and resulted in the destruction of millions of barberry bushes in the north-central U.S., particularly the spring wheat growing region. This program was so effective that many wheat growers alive now have never even seen stem rust. In addition, the pathogen population is nearly clonal with only a few races being collected each year in recent decades. Unfortunately, there is some evidence that the remnant barberry population is expanding and if this plant becomes common again, we will undoubtedly see increasing losses from stem rust due to a more diverse P. graminis population.

As noted in the section above, we are studying the efficacy of fungicides for stem rust. Based on preliminary field studies in 2008, it appears that wheat stem rust can be controlled using the same fungicide products as the other wheat rusts. However, further testing is needed especially in regards to application timing and product placement in the canopy. A figure summarizing these results can be obtained by clicking here.

Additional Images


 
Life cycle of Pgt
(from USDA-ARS-CDL)		 Severe stem rust on oats  

 

Links

References

Working on these.

* Please note that the author/host of this web site is located in South Dakota (USA) and this information might be region-centric.

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