Genetic structure of regional populations of Mycosphaerella graminicola (Septoria tritici), the septoria leaf blotch agent of wheat

Mycosphaerella graminicola (anamorph Septoria tritici ), the causal agent of septoria tritici blotch (STB) of wheat, is dominating species in Septoria / Stagonospora complex on crops in the main grain-producing areas of Russia. Resistance to STB may be either quantitative (horizomtal) or isolate-specific (vertical). At present 17 genes for resistance have been identified ( Stb1 - Stb17 ). The gen-for-gen interaction in the «wheat- M. graminicola » pathosystem has been demonstrated by genetic analysis; therefore, the aviability of resistance genes in the host proposes the existence of specific virulence genes in the pathogen. The relative frequency of virulence genes within a geographic region may be calculated as a fraction of the isolates expressing this virulence genes from the overall number of isolates used in the study. The purpose of the present study was to estimate the virulence genes in populations of M. graminicola from different geographic regions of Russia on the basis of a gen-for-gen relationship, using the cultivars with known resistance genes, i.e. Bulgaria 88 ( Stb1 ), Oasis ( Stb1 ), Veranopolis ( Stb 2), Israel 493 ( Stb 3), Tadinia ( Stb4 ), CS/Synthetic 7D ( Stb5 ), Flame ( Stb6 ), Estanzuela Federal ( Stb7 ), W7984 ( Stb8 ). A total of 47 isolates from the North-Caucasian region, 66 isolates from the Central-Chernozem region, 29 isolates from the Volga region, 64 isolates from the Central region, and 34 isolates from the North-West region were tested under greenhouse and grows chamber conditions. The virulence was estimated on seedlings at two-leaf stage, using two parameters, the infection degree of plants and sporulation of fungus in vivo. The effectiveness of Stb -genes to each regional population of M. graminicola was revealed on the basis of the frequency of virulence genes. The regional populations of M. graminicola differed in virulence genotype, spectrum and frequency of virulence genes. The populations from south zone (the North-Caucasian, the Central-Chernozem and the Volga regions) are more virulent in comparison with the central and the north-west populations. For example, 19.2 % of isolates from the north-caucasian population and 6.0 % of isolates from the central-chernozem population have no virulence genes, while in the central and north-west populations - 42.2 % and 44.1 %, respectively. Isolates from the Volga population of M. graminicola had most various combinations of virulence genes. High frequency of virulence to genes Stb1, Stb5 and Stb7 was revealed in all populations. The genes Stb2, Stb3, Stb4 have considerable effectiveness to the central, the central-chernozem and the north-west populations of M. graminicola, however it distinctly reduced concerning isolates from the North-Caucasian and the Volga regions. The genes Stb6 and Stb8 were highly effective ( Stb8 - absolutely effective) to all investigated Russian populations of M. graminicola and may be recommended for using in selection as sources of resistance to STB.