The Relationship Between Slow-Rusting and Some Genes Specific for Stem Rust Resistance in Wheat

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Abstract
The relationship between slow-rusting and genes for specific resistance against stem rust was studied in F5 lines derived from diallel crosses among the wheat [Triticum aestivum] cultivars ''Idaed 59'', ''Kenya 58'', ''Thatcher'', ''Lee'', ''Marquis'', ''Prelude'' and ''Baart''. Slow-rusting in the F5 lines was measured in natural epidemics of stem rust and by calculating the area under the rust progress curve derived from weekly estimates of stem rust severity. Lines that possessed either the dominant or recessive allele of genes for specific resistance, Sr5, Sr6, Sr7b, Sr11 and SrTtl, were identified in a seedling test in the greenhouse using appropriate cultures of Puccinia graminis f. sp. tritici. Then the slow-rusting ability of lines possessing the dominant allele of an Sr gene was compared to the slow-rusting ability of half-sib lines possessing the recessive allele of the same Sr gene. There was no effect on the development of stem rust attributable to the recessive or to the dominant alleles of Sr5, Sr7b and Sr11. There was an association between the development of stem rust and the dominant allele of the SrTtl gene, but slow-rusting was not due to this allele. The genes conditioning slow-rusting may be located on the same chromosome as the SrTtl locus and linked to SrTtl. There also was an interaction between slow-rusting and the dominant allele of gene Sr6. The F5 lines possessing the dominant allele of Sr6 rust more slowly than the lines possessing the recessive allele. However, in the group of lines with the dominant allele of Sr6, there were some fast-rusting lines. Slow-rusting was not due to the Sr6 gene, per se, but due to associated genes for slow-rusting.

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