Application of SSR markers for study of genetic diversity of Venturia inaequalis in the different types of orchards in the North Caucasian region

Apple scab caused by ascomycete fungus Venturia inaequalis (Cooke) G. Winter is one of the most harmful diseases of apple trees, which leads to significant economic losses in apple production in the world. North Caucasus is a region with climatic conditions favorable for V. inaequalis. Therefore, the creation of resistant varieties is an important target for apple breeding. Study of the genetic diversity of the pathogen is an integral part of both science-based apple breeding programs and systems of protection against the pathogen. This paper is the first report on SSR analysis of genetic diversity of V. inaequalis strains collected in apple orchards that differ in structure and are located geographically remotely in the Krasnodar Territory and the Republic of Adygea. To study the genetic polymorphism of the phytopathogen populations, two industrial gardens and a collection of Malus orientalis were surveyed in the Kuban and Caucasus foothill agro-ecological zones of the region. The genetic heterogeneity of the host plant populations at the sampling sites varied significantly, since the industrial orchards were single-cultivar plantations of the apple varieties Gala, Renet Simirenko, Golden Delicious, and Champion while in the collection garden the accessions originated from different parts of the M. orientalis natural area. Eight SSR markers used were 1aac4f, Viga7/116, Vitc1/2, Vitcca7/P, Vicacg8/42, Viga3/z, 1tcla, Vitc2/D. The number of alleles per locus revealed in SSR analysis of 36 monosporic isolates of V. inaequalis was 4 for 1aac4f, 6 for Vitc2/D, 10 for Viga7/116 and Vicacg8/42, 11 for Vitcca7/P, and 12 for Vitc1/2 and 1tcla. Upon the whole, there were 4 (1aac4f) to 12 alleles (Vitc1/2, 1tcla) for polymorphic markers, and only one allele was detected for marker Viga3/z. Despite the fact that some markers showed various distributions of identified alleles in all subpopulations, these differences were not sufficient to differentiate the subpopulations. UPGMA-analysis showed no relationship between clusterization and the geographical origin of the isolates, indicating low inter-population differences. This can indicate a free gene flow between the populations due to human activity as they are too distant from each other to allow natural transfer of spores. The obtained results suggest significant genetic diversity in the investigated set of monospore isolates. Genetic diversity was higher in the V. inaequalis population from the M. orientalis collection, indicating the effect of plant population heterogeneity on genetic polymorphism of the pathogen. In our opinion, the differences in polymorphism for some SSR markers, when compared our data and the results reported by other researchers' for European populations of V. inae-qualis, could be due to genetic differences in populations of V. inaequalis from North Caucasus region and the European populations.