Resistance to the downy mildew agent in the Eurasian grapevine germplasm of Georgia (Caucasus)

The oomycete Plasmopara viticola is the causal agent of downy mildew, one of the most important diseases affecting grapevine cultivation worldwide. P. viticola is an obligate parasite and biotrophic pathogen of members of the Vitaceae, introduced in Europe from North America in 1878. While it causes limited damages to American and Asian grapevine species, severe epidemics leading to yield losses are frequent in the Eurasian grapevine species, Vitis vinifera. The possibility that V. vinifera could be resistant to P. viticola has been largely unexplored in the past, due to absence of co-evolution with the pathogen and weak genetic diversity of the main cultivars. The recent discovery of the great genetic variability of the Caucasian grapevine, Georgian in particular, opened new perspectives on resistant traits in V. vinifera. A large collection of 600 V. vinifera accessions of Caucasian origin, cultivated and wild, has been phenotypically characterized for resistance to P. viticola through experimental inoculation in laboratory and disease assessment in open field. This led to the discovery of 33 accessions with a resistance level from medium to high, all from Georgia. The Georgian accession, Mgaloblishvili, in particular, showed unique resistance traits based on comparative transcriptomic analysis and confocal microscopy analyses. In conclusion, the cradle of viticulture, located in Georgia (Caucasus), represents an important source of resistance to P. viticola that could be exploited for genetic improvement of grapevine, one of the most specific, environmentally safe and innovative tools for plant disease management.