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MF2989 Identification and Management of Stem Rust On Identification and Management of Stem Rust on Wheat and Barley Stem rust, leaf rust, and stripe rust comprise a production. The first of these races, known as ‘Ug99’, complex of diseases that reduces wheat and barley was originally detected in Uganda, Kenya, and grain production. These rust diseases occur in nearly Ethiopia. Since this initial detection, additional races all areas of the United States and Canada. The of the fungus have been reported and are further importance of any member of the complex at a given complicating efforts to contain the problem. location is determined by specific interactions with If these new races spread to North America, current wheat varieties, they may threaten wheat crop growth stage, and and barley production. weather conditions. Why does the genetic In preparation for the Stem rust has been resistance appear to fail? possible introduction of present in North America Currently genetic resistance is an effective these new races of stem for hundreds of years. means of managing the rust diseases of rust, a number of critical A series of particularly wheat and barley. Because the populations questions arise regarding severe outbreaks occurred the most effective ways during the early 1930s and of the fungi that cause rust diseases can to identify, monitor, and 1950s. These outbreaks change and adapt to the resistance genes of manage the disease. This caused serious yield loss in current varieties, the durability of this genetic publication answers these many parts of the United resistance has been problematic. These critical questions with the States and Canada with changes occur when naturally occurring best available information the greatest losses in the genetic changes allow members of the about the emerging threat. Great Plains. More local- fungal population to overcome the genetic ized outbreaks of wheat What is the regional stem rust occurred in the resistance of the plant. The rust fungi have a risk of stem rust southern Great Plains as tremendous reproduction potential and are epidemics? recently as 1986. Stem rust easily moved by wind; therefore, a new race If stem rust reemerges became a problem in barley of stem rust often increases rapidly, resulting as a serious problem in in the late 1980s and early in outbreaks of disease throughout a large North America, historical 1990s. In all these cases, geographical region. accounts of stem rust the increase in frequency outbreaks will provide and intensity of the disease important information was associated with about where the disease is changes in the population of the stem rust fungus, likely to be most severe and cause the largest reduc- which enabled it to overcome genetic resistance of tions in yield. Stem rust normally survives the winter common varieties. months in the southern United States and Mexico. After several decades of control with genetic The disease then becomes established near overwin- resistance, new genetic variants (races) of the stem tering locations in southern states and is subsequently rust fungus have emerged as a threat to wheat moved north by weather systems. Potential yield loss in northern states is strongly influenced by the varieties. The magnitude of this change in disease intensity of the disease outbreaks in southern states resistance depends on the diversity of the new and the timing of these northerly movements of the population of stem rust, specific combinations of disease, relative to crop resistance genes present development. Understanding the terminology within each variety, Based on the and interactions with historical accounts of of rust resistance genes affected the environment. stem rust outbreaks, it by ‘Ug99’ Until the exact appears that in most The populations of stem rust causing problems reactions to the new years the disease may in other parts of the world can be grouped by the races of stem rust can develop too late to resistance genes that they are able to overcome. be determined, it may cause much yield loss in These stem rust resistance genes are generally be useful to group Texas and Oklahoma. varieties based on the Moderate yield losses designated by an abbreviation for stem rust (Sr) stem rust resistance could occur in Kansas and a unique code of letters and numbers that genes they contain. If and Nebraska, but the help scientists communicate more effectively stem rust overcomes an greatest losses are most when breeding new varieties for disease resistance. important resistance likely to occur in South The original ‘Ug99’ is able to overcome the gene, all varieties Dakota, North Dakota, stem rust resistance gene Sr31. Since this initial protected by that gene Minnesota, Manitoba, discovery, additional types of the fungus have will likely experience and Saskatchewan significant reductions (Figure 1). Historical been discovered that are able to overcome the in stem rust resistance. reports of stem rust resistance genes Sr24, and Sr36. These resistance Varieties within other outbreaks indicate a genes are the basis for the genetic vulnerability groups, which have lower risk of yield loss that has scientists around the world concerned other resistance genes, east of the Mississippi about potential outbreaks of stem rust. may remain unaffected. River and in the Pacific Scientists around Northwest. the world are working Although this to incorporate new historical information provides some useful insights sources of resistance that will reduce the risk of into where stem rust is most likely to be a problem, regional disease outbreaks. Until these sources of recent changes in agricultural production may influ- resistance become widely available, farmers should ence the current risk of disease (Table 1). In most evaluate their regional risk of disease and carefully cases, current agricultural practices will diminish evaluate the sources of genetic resistance used on the risk of severe stem rust relative to historical their farm. Planting varieties with different sources outbreaks of disease. of resistance may help reduce the potential that any How will current wheat single race of stem rust will affect all varieties on a and barley varieties respond farm. Information regarding genetic resistance of to the new types of stem rust? many common wheat and barley varieties can be found on the USDA’s Cereal Disease Laboratory The introduction of one or more new races of website: www.ars.usda.gov/mwa/cdl. stem rust into North America might cause dramatic changes in the stem rust reaction of many wheat Table 1. Factors influencing the risk of stem rust and severe yield losses relative to historical outbreaks of the disease. Change to the production system Potential affect on stem rust outbreaks In many areas, less land is used for wheat and barley The distance between fields may cause the disease to production. These areas of cultivation are more spread more slowly and reduce the risk of movement isolated from each other. between regions. Areas away from main areas of production may escape severe disease. Wheat and barley production have moved to areas Growing wheat and barley in these drier climates with historically drier climates. may reduce the risk of disease development. Many wheat varieties mature earlier, relative to those Yield losses to stem rust are greatest when severe that were grown a few decades ago. disease develops during the early stages of grain development. Varieties that mature earlier may escape serious yield losses. Access to highly effective fungicides and application It may be possible to reduce the influence of the technologies disease on yield and slow regional spread of stem rust with fungicides. Figure 1. Estimated risk of yield losses caused by stem rust in North America based on historical records of disease outbreaks. Risk Map Legend High risk: disease outbreaks and severe yield losses are likely Moderate risk: disease outbreaks and moderate yield losses are possible Low risk: disease outbreaks causing moderate yield losses are rare No historical information Based on information summarized by Leonard, K. J. 2001. In P.D. Peterson (ed) Stem rust of wheat: from ancient enemy to modern foe. APS Press, St. Paul, MN. Figure 2. Key diagnostic features of stem rust. A B C D Leaf rust A. Stem rust commonly infects stems, leaves, Stem rust and leaf sheaths. B. Stem rust occasionally infects parts of the head including glumes and awns. C. Stem rust causes oval-shaped or elongated blister-like lesions. Tearing or ripping of the outer layers of the plant tissue is visible without magnification. D. Stem rust lesions tend to be larger and longer than those caused by leaf rust. How is stem rust identified? attention of disease specialists. The disease specialist Using several key diagnostic features, stem rust will make a preliminary evaluation of the sample and can be distinguished from other rust diseases of wheat. send suspect samples to the experts at the USDA’s When severe, stem rust can be differentiated from leaf Cereal Disease Laboratory in St. Paul, Minnesota or rust or stripe rust by the parts of the plant that are AAFC Cereal Disease Research Center in Winnipeg, infected. Stem rust is able to infect the stems, leaves, Manitoba to confirm the diagnosis. Disease specialists and leaf sheaths of the plant (Figure 2). This infec- in your state or province can be found through your tion of the stem helps separate it from the other rust local extension office or by contacting a diagnostic diseases. Leaf rust can infect the leaf sheath that wraps laboratory near you. Contact information is provided at around the stem, but not the stem itself. the end of this publication. When the disease is less severe and only a few How much yield loss can stem rust cause? lesions can be found, it is important to focus on specific characteristics of the lesions. Stem rust causes Stem rust is generally considered the most elongated or oval-shaped blister-like lesions.
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