Presence of the Stb6 gene for resistance to septoria tritici blotch (Mycosphaerella graminicola) in cultivars used in wheat-breeding programmes worldwide

Fifty-two wheat cultivars and breeding lines, most of which have been used in breeding programmes worldwide, were tested for isolate-specific resistance to Mycosphaerella graminicola isolate IPO323, which interacts with the Stb6 gene of wheat (first identified in cvs Flame and Hereward) via a gene-for-gene relationship. Twenty-three lines were specifically resistant to this isolate. Sixteen resistant lines were crossed with Flame for a test of allelism. All progeny lines were resistant, suggesting that the 16 parental lines had Stb6, a gene allelic to it or a gene closely linked to it. In 14 lines, resistance to IPO323 was controlled by Stb6 only. An exception was Kavkaz-K4500 L6.A.4., which has two genes for resistance to IPO323, one of which is Stb6. The microsatellite marker Xgwm369 was used to examine genetic diversity in the region of the genome containing Stb6, to which it is closely linked. Eleven alleles of Xgwm369, with amplified fragments of 10 different sizes, as well as apparent nonamplification of this marker in Bulgaria 88, were detected. Through the use of information about lines' ancestry, combined with Xgwm369 alleles, it was shown that Stb6 entered world wheat-breeding programmes on a minimum of six occasions, and possibly from as many as 11 sources. The presence of Stb6 in both European and Chinese landraces suggests that this gene has been present in cultivated wheat since the earliest times of agriculture.