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Septoria tritici blotch (STB), caused by Mycosphaerella graminicola (synonym: Zymoseptoria tritici; asexual stage: Septoria tritici), is an important disease of wheat worldwide. Management of the disease usually is by host resistance or fungicides. However, M. graminicola has developed insensitivity to most commonly applied fungicides so there is a continuing need for well-characterized sources of host resistance to accelerate the development of improved wheat cultivars. Gene Stb3 has been a useful source of major resistance, but its mapping location has not been well characterized. Based on linkage to a single marker, a previous study assigned Stb3 to a location on the short arm of chromosome 6D. However, the results from the present study show that this reported location is incorrect. Instead, linkage analysis revealed that Stb3 is located on the short arm of wheat chromosome 7A, completely linked to microsatellite (SSR) locus Xwmc83 and flanked by loci Xcfa2028 (12.4 cM distal) and Xbarc222 (2.1 cM proximal). Linkage between Stb3 and Xwmc83 was validated in BC1F3 progeny of other crosses, and analyses of the flanking markers with deletion stocks showed that the gene is located on 7AS between fraction lengths 0.73 and 0.83. This revised location of Stb3 is different from those for other STB resistance genes previously mapped in hexaploid wheat but is approximately 20 cM proximal to an STB resistance gene mapped on the short arm of chromosome 7Am in Triticum monococcum. The markers described in this study are useful for accelerating the deployment of Stb3 in wheat breeding programs.

Septoria tritici blotch (STB), caused by Mycosphaerella graminicola (synonym: Zymoseptoria tritici; asexual stage: Septoria tritici), is an important disease of wheat worldwide. Management of the disease usually is by host resistance or fungicides. However, M. graminicola has developed insensitivity to most commonly applied fungicides so there is a continuing need for well-characterized sources of host resistance to accelerate the development of improved wheat cultivars. Gene Stb3 has been a useful source of major resistance, but its mapping location has not been well characterized. Based on linkage to a single marker, a previous study assigned Stb3 to a location on the short arm of chromosome 6D. However, the results from the present study show that this reported location is incorrect. Instead, linkage analysis revealed that Stb3 is located on the short arm of wheat chromosome 7A, completely linked to microsatellite (SSR) locus Xwmc83 and flanked by loci Xcfa2028 (12.4 cM distal) and Xbarc222 (2.1 cM proximal). Linkage between Stb3 and Xwmc83 was validated in BC1F3 progeny of other crosses, and analyses of the flanking markers with deletion stocks showed that the gene is located on 7AS between fraction lengths 0.73 and 0.83. This revised location of Stb3 is different from those for other STB resistance genes previously mapped in hexaploid wheat but is approximately 20 cM proximal to an STB resistance gene mapped on the short arm of chromosome 7Am in Triticum monococcum. The markers described in this study are useful for accelerating the deployment of Stb3 in wheat breeding programs.

ABSTRACT Breeding wheat for resistance is the most effective means to control Septoria tritici blotch (STB), caused by the ascomycete Mycosphaerella graminicola (anamorph Septoria tritici). At least eight genes that confer resistance to STB in wheat have been identified. Among them, the Stb4 locus from the wheat cv. Tadinia showed resistance to M. graminicola at both seedling and adult-plant stages. However, no attempt has been made to map the Stb4 locus in the wheat genome. A mapping population of 77 F10 recombinant-inbred lines (RILs) derived from a three-way cross between the resistant cv. Tadinia and the susceptible parent (Yecora Rojo x UC554) was evaluated for disease resistance and molecular mapping. The RILs were tested with Argentina isolate I 89 of M. graminicola for one greenhouse season in Brazil during 1999, with an isolate from Brazil (IPBr1) for one field season in Piracicaba (Brazil) during 2000, and with Indiana tester isolate IN95-Lafayette-1196-WW-1-4 in the greenhouse during 2000 and 2001. The ratio of resistant:susceptible RILs was 1:1 in all three tests, confirming the single-gene model for control of resistance to STB in Tadinia. However, the patterns of resistance and susceptibility were different between the Indiana isolate and those from South America. For example, the ratio of RILs resistant to both the Indiana and Argentina isolates, resistant to one but susceptible to the other, and susceptible to both isolates was approximately 1:1:1:1, indicating that Tadinia may contain at least two genes for resistance to STB. A similar pattern was observed between the Indiana and Brazil isolates. The gene identified with the Indiana tester isolate was assumed to be the same as Stb4, whereas that revealed by the South American isolates may be new. Bulked-segregant analysis was used to identify amplified fragment length polymorphism (AFLP) and microsatellite markers linked to the presumed Stb4 gene. The AFLP marker EcoRI-ACTG/MseI-CAAA5 and microsatellite Xgwm111 were closely linked to the Stb4 locus in coupling at distances of 2.1 and 0.7 centimorgans (cM), respectively. A flanking marker, AFLP EAGG/ M-CAT10, was 4 cM from Stb4. The Stb4 gene was in a potential supercluster of resistance genes near the centromere on the short arm of wheat chromosome 7D that also contained Stb5 plus five previously identified genes for resistance to Russian wheat aphid. The microsatellite marker Xgwm111 identified in this study may be useful for facilitating the transfer of Stb4 into improved cultivars of wheat.