Host Plant Resistance Genes for Fusarium Head Blight: Sources, Mechanisms, and Utility in Conventional Breeding Systems

source of complete resistance is known, and current sources provide only partial resistance. Fusarium head blight (FHB), caused by Fusarium graminearum The United States Department of Agriculture (USDA) Schwabe [teleomorph Gibberella zeae (Schwein.)], also known as scab, is a destructive disease of wheat (Triticum aestivum L; T. turcurrently ranks FHB as the worst plant disease of wheat gidum L. var durum ) and barley (Hordeum vulgare L.). Host resisand barley since the stem rust (caused by Puccinia gramtance has long been considered the most practical and effective means inis Pers.:Pers.) epidemics of the 1950s (Wood et al., of control, but breeding has been hindered by a lack of effective 1999). FHB epidemics have been documented in 26 resistance genes and by the complexity of the resistance in identified states and five Canadian provinces. Yield losses in wheat sources. This paper will provide an overview of progress in developing since 1990 have exceeded 13 Tg (500 million bushels) host plant resistance for FHB, primarily in the USA, by review of with economic losses estimated at $2.5 billion (Windels, the sources of resistance in wheat and barley, and their utilization 2000). Wheat yields in 1993 were reduced by about 50% in breeding programs. Although there are no reported sources of in northeastern North Dakota and 40% in northwestern immunity, considerable genetic variability exists for resistance in both wheat and barley. Sources of resistance in durum, however, are limMinnesota compared with 1992 (National Agricultural ited. The strategy of breeding programs is to recombine different Statistics Service, 1993–1999). In barley, losses have types and sources of resistance steadily through traditional breeding been equally devastating with estimated losses from strategies. To facilitate selection, artificial inoculation techniques are 1993 to 1999 totaling in excess of $400 million (Windels, used in both the field and greenhouse. This enables breeders to select 2000). Since 1993, North Dakota, South Dakota, and simultaneously for resistance and desirable agronomic characteristics. Minnesota have lost 73% of their malting barley market Incremental increases in resistance are being reported in hexaploid with losses in Minnesota alone approaching 95% (Winwheat and to a lesser extent in barley and durum wheat. It is anticidels, 2000). pated that the development of molecular markers will improve the The resurgence of FHB has united scientists, producefficiency of developing FHB wheat and barley cultivars. ers, industry, and politicians in an effort to find a solution to this disease. Nearly $10 million in new public and private funding has been generated to support FHB F usarium graminearum Schwabe [teleomorph Gibresearch and education in the USA (Windels, 2000). In berella zeae (Schwein.)], also known as Fusarium 1999, the U.S. Wheat and Barley Scab Initiative suphead blight or scab is a destructive disease of wheat and ported research efforts of 66 principal scientists working barley in warm and humid wheat growing regions of on 111 research projects at 19 Land Grant Universities the world. FHB has recently gained prominence as a and the USDA Agricultural Research Service (U.S. national research problem in the USA because of the Wheat and Barley Scab Initiative news release, http:// more frequent U.S. epiphytotics thought to be the result www.scabusa.org/newpage13.htm, 1 Dec. 2000). of the increased emphasis on conservation tillage (Bai Scab is not a new problem. As early as 1891, Arthur and Shaner, 1994; Wilcoxson et al., 1988), rotations with (1891) stressed the importance of breeding for resiscorn (Zea mays L.) (Windels and Kommedahl, 1984), tance to head blight in wheat. In the 1920s, plant paththe lack of effective cultural and/or fungicide control ologists and breeders observed that wheat genotypes (McMullen et al., 1997), above average precipitation differed in their susceptibility to FHB; however, differand/or humidity during flowering and early grain fill ences in maturity made it hard to separate genotypic (McMullen et al., 1997), and the lack of effective sources differences in susceptibility from disease escape (Immer of genetic resistance. In addition to reduced kernel denand Christensen, 1943). Much of our present knowledge sity and discoloration at harvest, associated deoxynivaliabout FHB originates from extensive research that was nol (DON) accumulation prevents grain from being done at the University of Minnesota from the 1920s marketable. Host resistance has long been considered through the 1950s (Hanson et al., 1950; Schroeder and the most practical and effective means of control (MarChristensen, 1963). A severe rust epidemic, however, tin and Johnston, 1982; Schroeder and Christensen, changed the focus of wheat research to stem rust and 1963), but breeding has been hindered by a lack of U.S. work on FHB was discontinued (Wilcoxson, 1993). effective resistance genes and by the complexity of the Worldwide, resistance to FHB is a major focus of resistance in identified sources (Mesterházy, 1997). No wheat and barley breeding programs. In China, FHB research began in the 1950s and continues today (Liu J.C. Rudd, Plant Science Dep., South Dakota State Univ., Brookings, and Wang, 1990). Japanese scientists initiated a major SD 57006; R.D. Horsley and E.M. Elias, Dep. of Plant Sciences, North Dakota State Univ., Fargo, ND 58105; A.L. McKendry, Dep. Abbreviations: AFLP, amplified fragment length polymorphism; of Agronomy, Univ. of Missouri, Columbia, MO 65211. Received 28 DON, deoxynivalenol; FHB, Fusarium head blight; QTL, quantitative March 2000. *Corresponding author ([email protected]). trait locus; RFLP, restriction fragment length polymorphism; RIL, recombinant inbred line; SSR, single sequence repeat. Published in Crop Sci. 41:620–627 (2001).