Cryopreservation as part of the conservation strategy of <i>Fragaria</i> germplasm in Germany

In Germany, genetic resources of the genus Fragaria are predominantly represented by the 267 strawberry cultivars of the German Fruit Genebank, which is a decentralized network of different stakeholders, and the comprehensive wild species collection of the Fruit genebank of the Julius Kühn Institute (JKI) in Dresden (288 accessions belonging to 22 species). Strawberry is a vegetatively propagated fruit crop. Maintaining the genetic diversity of these crops is more challenging than for most seed-producing crops, as the unique heterozygosity of genotypes identified and selected for their particular combination of genetic traits, need to be maintained. Primary collections of Fragaria are usually maintained in field collections. Duplication of germplasm is necessary to prevent complete loss of genetic resources due to biotic and abiotic disasters. This can be any useful combination of field cultivation at sites other than the main collection, in vitro culture or cryopreservation. However, cryopreservation has comprehensive advantages, as this form of storage offers security against loss through plant diseases or environmental influences. For this reason, a PVS2 vitrification method with a 14-d alternating-temperature cold acclimation, was developed and used for screening of 188 strawberry cultivars and 22 Fragaria wild species (56 accessions) at the JKI in Dresden. The average recovery for the cultivars was 87.23% and for the wild species 85.34%. In parallel, virus-free maintenance was established in the screen house. This stock consists of virus free genotypes based on heat and/or cryotherapy and testing of four viruses. In addition, seeds of wild strawberry species native to Germany are produced by selective crossing and sent for storage to the Svalbard Global Seed Vault in Norway.