Abstract
Loose smut, caused by Ustilago tritici (Pers.) Rostr., is a systemic disease of tetraploid durum wheat (Triticum turgidum L.). Loose smut can be economically controlled by growing resistant varieties, making it important to find and deploy new sources of resistance. Blackbird, a variety of T. turgidum L. subsp. carthlicum (Nevski) A. Love & D. Love, carries a high level of resistance to loose smut. Blackbird was crossed with the loose smut susceptible durum cultivar Strongfield to produce a doubled haploid (DH) mapping population. The parents and progenies were inoculated with U. tritici races T26, T32 and T33 individually and as a mixture at Swift Current, Canada in 2011 and 2012 and loose smut incidence (LSI) was assessed. Genotyping of the DH population and parents using an Infinium iSelect 90K single nucleotide polymorphism (SNP) array identified 12,952 polymorphic SNPs. The SNPs and 426 SSRs (previously genotyped in the same population) were mapped to 16 linkage groups spanning 3008.4 cM at an average inter-marker space of 0.2 cM in a high-density genetic map. Composite interval mapping analysis revealed three significant quantitative trait loci (QTL) for loose smut resistance on chromosomes 3A, 6B and 7A. The loose smut resistance QTL on 6B (QUt.spa-6B.2) and 7A (QUt.spa-7A.2) were derived from Blackbird. Strongfield contributed the minor QTL on 3A (QUt.spa-3A.2). The resistance on 6B was a stable major QTL effective against all individual races and the mixture of the three races; it explained up to 74% of the phenotypic variation. This study is the first attempt in durum wheat to identify and map loose smut resistance QTL using a high-density genetic map. The QTL QUt.spa-6B.2 would be an effective source for breeding resistance to multiple races of the loose smut pathogen because it provides near-complete broad resistance to the predominant virulence on the Canadian prairies.
Highlights
Durum wheat [Triticum turgidum L. subsp. durum (Desf.) Husn. (2n = 4× = 28 AABB genome)] is an important component of the diet of people in many regions of the world
The resistant parent Blackbird showed complete to nearly complete resistance to individual and mixed inoculation of races T26, T32 and T33 (Figs 1 and 2)
Susceptible parent Strongfield was moderately resistant to race T26 but highly susceptible to individual races T32 and T33, while it was found to be susceptible to the mixture of races T26, T32 and T33 (S1 Table)
Summary
Most of the durum wheat cultivars registered for production on the Canadian prairies are susceptible to many races of the loose smut pathogen [3, 4]. Loose smut is caused by the fungus Ustilago tritici (Persoon) Rostrup, a seed-borne pathogen. It replaces the spike floral tissues with dark brown masses of teliospores, causing yield reduction approximately proportional to the percentage of smutted spikes. Loose smut resistant wheat varieties have been developed and grown, very little effort has been applied to the identification and mapping of genomic regions controlling infection of U. tritici races. Identification of favorable alleles and incorporation of loose smut resistance in high yielding, well-adapted wheat cultivars is desirable to reduce the need for chemical control measures and thereby reduce the environmental footprint
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